htmLawed documentation

1.1  Example uses

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  *  Filtering of text submitted as comments on blogs to allow only certain HTML elements

  *  Making RSS/Atom newsfeed item-content standard-compliant: often one uses an excerpt from an HTML document for the content, and with unbalanced tags, non-numerical entities, etc., such excerpts may not be XML-compliant

  *  Text processing for stricter XML standard-compliance: e.g., to have lowercased x in hexadecimal numeric entities becomes necessary if an XHTML document with MathML content needs to be served as application/xml

  *  Scraping text or data from web-pages

  *  Pretty-printing HTML code

1.2  Features

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  Key: * security feature, ^ standard compliance, ~ requires setting right options, ` different from Kses

  *  make input more secure and standard-compliant
  *  use for HTML 4, XHTML 1.0 or 1.1, or even generic XML documents  ^~`

  *  beautify or compact HTML  ^~`

  *  restrict elements  ^~`
  *  proper closure of empty elements like img  ^`
  *  transform deprecated elements like u  ^~`
  *  HTML comments and CDATA sections can be permitted  ^~`
  *  elements like script, object and form can be permitted  ~

  *  restrict attributes, including element-specifically  ^~`
  *  remove invalid attributes  ^`
  *  element and attribute names are lower-cased  ^
  *  provide required attributes, like alt for image  ^`
  *  transform deprecated attributes  ^~`
  *  attributes declared only once  ^`

  *  restrict attribute values, including element-specifically  ^~`
  *  a value is declared for empty (minimized) attributes like checked  ^
  *  check for potentially dangerous attribute values  *~
  *  ensure unique id attribute values  ^~`
  *  double-quote attribute values  ^
  *  lower-case standard attribute values like password  ^`

  *  attribute-specific URL protocol/scheme restriction  *~`
  *  disable dynamic expressions in style values  *~`

  *  neutralize invalid named character entities  ^`
  *  convert hexadecimal numeric entities to decimal ones, or vice versa  ^~`
  *  convert named entities to numeric ones for generic XML use  ^~`

  *  remove null characters  *
  *  neutralize potentially dangerous proprietary Netscape Javascript entities  *
  *  replace potentially dangerous soft-hyphen character in attribute values with spaces  *

  *  remove common invalid characters not allowed in HTML or XML  ^`
  *  replace characters from Microsoft applications like Word that are discouraged in HTML or XML  ^~`
  *  neutralize entities for characters invalid or discouraged in HTML or XML  ^`
  *  appropriately neutralize <, &, ", and > characters  ^*`

  *  understands improperly spaced tag content (like, spread over more than a line) and properly spaces them  `
  *  attempts to balance tags for well-formedness  ^~`
  *  attempts to permit only validly nested tags  ^~`
  *  option to remove or neutralize bad content ^~`
  *  attempts to rectify common errors of plain-text misplacement (e.g., directly inside blockquote) ^~`

  *  fast, non-OOP code of ~45 kb incurring peak basal memory usage of ~0.5 MB
  *  compatible with pre-existing code using Kses (the filter used by WordPress)

  *  optional anti-spam measures such as addition of rel="nofollow" and link-disabling  ~`
  *  optionally makes relative URLs absolute, and vice versa  ~`

  *  optionally mark & to identify the entities for &, < and > introduced by htmLawed  ~`

  *  allows deployment of powerful hook functions to inject HTML, consolidate style attributes to class, finely check attribute values, etc.  ~`

  *  independent of character encoding of input and does not affect it

  *  tolerance for ill-written HTML to a certain degree

1.3  History

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  htmLawed was developed for use with LabWiki, a wiki software developed at PHP Labware, as a suitable software could not be found. Existing PHP software like Kses and HTMLPurifier were deemed inadequate, slow, resource-intensive, or dependent on external applications like HTML Tidy.

  htmLawed started as a modification of Ulf Harnhammar's Kses (version 0.2.2) software, and is compatible with code that uses Kses; see section 2.6.

1.4  License & copyright

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  htmLawed is free and open-source software licensed under GPL license version 3, and copyrighted by Santosh Patnaik, MD, PhD.

1.5  Terms used here

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  *  administrator - or admin; person setting up the code to pass input through htmLawed; also, user
  *  attributes - name-value pairs like href="http://x.com" in opening tags
  *  author - writer
  *  character - atomic unit of text; internally represented by a numeric code-point as specified by the encoding or charset in use
  *  entity - markup like &gt; and &#160; used to refer to a character
  *  element - HTML element like a and img
  *  element content -  content between the opening and closing tags of an element, like click of <a href="x">click</a>
  *  HTML - implies XHTML unless specified otherwise
  *  input - text string given to htmLawed to process
  *  processing - involves filtering, correction, etc., of input
  *  safe - absence or reduction of certain characters and HTML elements and attributes in the input that can otherwise potentially and circumstantially expose web-site users to security vulnerabilities like cross-site scripting attacks (XSS)
  *  scheme - URL protocol like http and ftp
  *  specs - standard specifications
  *  style property - terms like border and height for which declarations are made in values for the style attribute of elements
  *  tag - markers like <a href="x"> and </a> delineating element content; the opening tag can contain attributes
  *  tag content - consists of tag markers < and >, element names like div, and possibly attributes
  *  user - administrator
  *  writer - end-user like a blog commenter providing the input that is to be processed; also, author

2.1  Simple

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  The input text to be processed, $text, is passed as an argument of type string; htmLawed() returns the processed string:

    $processed = htmLawed($text);

  Note: If input is from a $_GET or $_POST value, and magic quotes are enabled on the PHP setup, run stripslashes() on the input before passing to htmLawed.

  By default, htmLawed will process the text allowing all valid HTML elements/tags, secure URL scheme/CSS style properties, etc. It will allow CDATA sections and HTML comments, balance tags, and ensure proper nesting of elements. Such actions can be configured using two other optional arguments -- $config and $spec:

    $processed = htmLawed($text, $config, $spec);

  These extra parameters are detailed below. Some examples are shown in section 2.9.

  Note: For maximum protection against XSS and other scripting attacks (e.g., by disallowing Javascript code), consider using the safe parameter; see section 3.6.

2.2  Configuring htmLawed using the $config parameter

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  $config instructs htmLawed on how to tackle certain tasks. When $config is not specified, or not set as an array (e.g., $config = 1), htmLawed will take default actions. One or many of the task-action or value-specification pairs can be specified in $config as array key-value pairs. If a parameter is not specified, htmLawed will use the default value/action indicated further below.

    $config = array('comment'=>0, 'cdata'=>1);
    $processed = htmLawed($text, $config);

  Or,

    $processed = htmLawed($text, array('comment'=>0, 'cdata'=>1));

  Below are the possible value-specification combinations. In PHP code, values that are integers should not be quoted and should be used as numeric types (unless meant as string/text).

  Key: * default, ^ different default when htmLawed is used in the Kses-compatible mode (see section 2.6), ~ different default when valid_xhtml is set to 1 (see section 3.5), " different default when safe is set to 1 (see section 3.6)

  abs_url
  Make URLs absolute or relative; $config["base_url"] needs to be set; see section 3.4.4

  -1 - make relative
  0 - no action  *
  1 - make absolute

  and_mark
  Mark & characters in the original input; see section 3.2

  anti_link_spam
  Anti-link-spam measure; see section 3.4.7

  0 - no measure taken  *
  array("regex1", "regex2") - will ensure a rel attribute with nofollow in its value in case the href attribute value matches the regular expression pattern regex1, and/or will remove href if its value matches the regular expression pattern regex2. E.g., array("/./", "/://\W*(?!(abc\.com|xyz\.org))/"); see section 3.4.7 for more.

  anti_mail_spam
  Anti-mail-spam measure; see section 3.4.7

  0 - no measure taken  *
  word - @ in mail address in href attribute value is replaced with specified word

  balance
  Balance tags for well-formedness and proper nesting; see section 3.3.3

  0 - no
  1 - yes  *

  base_url
  Base URL value that needs to be set if $config["abs_url"] is not 0; see section 3.4.4

  cdata
  Handling of CDATA sections; see section 3.3.1

  0 - don't consider CDATA sections as markup and proceed as if plain text  ^"
  1 - remove
  2 - allow, but neutralize any <, >, and & inside by converting them to named entities
  3 - allow  *

  clean_ms_char
  Replace discouraged characters introduced by Microsoft Word, etc.; see section 3.1

  0 - no  *
  1 - yes
  2 - yes, but replace special single & double quotes with ordinary ones

  comment
  Handling of HTML comments; see section 3.3.1

  0 - don't consider comments as markup and proceed as if plain text  ^"
  1 - remove
  2 - allow, but neutralize any <, >, and & inside by converting to named entities
  3 - allow  *

  css_expression
  Allow dynamic CSS expression by not removing the expression from CSS property values in style attributes; see section 3.4.7

  0 - remove  *
  1 - allow

  deny_attribute
  Denied HTML attributes; see section 3.4

  0 - none  *
  string - dictated by values in string
  on* (like onfocus) attributes not allowed - "

  elements
  Allowed HTML elements; see section 3.3

  * -center -dir -font -isindex -menu -s -strike -u -  ~
  applet, embed, iframe, object, script not allowed - "

  hexdec_entity
  Allow hexadecimal numeric entities and do not convert to the more widely accepted decimal ones, or convert decimal to hexadecimal ones; see section 3.2

  0 - no
  1 - yes  *
  2 - convert decimal to hexadecimal ones

  hook
  Name of an optional hook function to alter the input string, $config or $spec before htmLawed starts its main work; see section 3.7

  0 - no hook function  *
  name - name is name of the hook function (kses_hook  ^)

  hook_tag
  Name of an optional hook function to alter tag content finalized by htmLawed; see section 3.4.9

  0 - no hook function  *
  name - name is name of the hook function

  keep_bad
  Neutralize bad tags by converting < and > to entities, or remove them; see section 3.3.3

  0 - remove  ^
  1 - neutralize both tags and element content
  2 - remove tags but neutralize element content
  3 and 4 - like 1 and 2 but remove if text (pcdata) is invalid in parent element
  5 and 6 * -  like 3 and 4 but line-breaks, tabs and spaces are left

  lc_std_val
  For XHTML compliance, predefined, standard attribute values, like get for the method attribute of form, must be lowercased; see section 3.4.5

  0 - no
  1 - yes  *

  make_tag_strict
  Transform/remove these non-strict XHTML elements, even if they are allowed by the admin: applet center dir embed font isindex menu s strike u; see section 3.3.2

  0 - no  ^
  1 - yes, but leave applet, embed and isindex elements that currently can't be transformed  *
  2 - yes, removing applet, embed and isindex elements and their contents (nested elements remain)  ~

  named_entity
  Allow non-universal named HTML entities, or convert to numeric ones; see section 3.2

  0 - convert
  1 - allow  *

  no_deprecated_attr
  Allow deprecated attributes or transform them; see section 3.4.6

  0 - allow  ^
  1 - transform, but name attributes for a and map are retained  *
  2 - transform

  parent
  Name of the parent element, possibly imagined, that will hold the input; see section 3.3

  safe
  Magic parameter to make input the most secure against XSS without needing to specify other relevant $config parameters; see section 3.6

  0 - no  *
  1 - will auto-adjust other relevant $config parameters (indicated by " in this list)

  schemes
  Array of attribute-specific, comma-separated, lower-cased list of schemes (protocols) allowed in attributes accepting URLs; * covers all unspecified attributes; see section 3.4.3

  href: aim, feed, file, ftp, gopher, http, https, irc, mailto, news, nntp, sftp, ssh, telnet; *:file, http, https  *
  *: ftp, gopher, http, https, mailto, news, nntp, telnet  ^
  href: aim, feed, file, ftp, gopher, http, https, irc, mailto, news, nntp, sftp, ssh, telnet; style: nil; *:file, http, https  "

  show_setting
  Name of a PHP variable to assign the finalized $config and $spec values; see section 3.8

  tidy
  Beautify or compact HTML code; see section 3.3.5

  -1 - compact
  0 - no  *
  1 or string - beautify (custom format specified by string)

  unique_ids
  id attribute value checks; see section 3.4.2

  0 - no  ^
  1 - remove duplicate and/or invalid ones  *
  word - remove invalid ones and replace duplicate ones with new and unique ones based on the word; the admin-specified word, like my_, should begin with a letter (a-z) and can contain letters, digits, ., _, -, and :.

  valid_xhtml
  Magic parameter to make input the most valid XHTML without needing to specify other relevant $config parameters; see section 3.5

  0 - no  *
  1 - will auto-adjust other relevant $config parameters (indicated by ~ in this list)

  xml:lang
  Auto-adding xml:lang attribute; see section 3.4.1

  0 - no  *
  1 - add if lang attribute is present
  2 - add if lang attribute is present, and remove lang  ~

2.3  Extra HTML specifications using the $spec parameter

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  The $spec argument can be used to disallow an otherwise legal attribute for an element, or to restrict the attribute's values. This can also be helpful as a security measure (e.g., in certain versions of browsers, certain values can cause buffer overflows and denial of service attacks), or in enforcing admin policy compliance. $spec is specified as a string of text containing one or more rules, with multiple rules separated from each other by a semi-colon (;). E.g.,

    $spec = 'i=-*; td, tr=style, id, -*; a=id(match="/[a-z][a-z\d.:\-`"]*/i"/minval=2), href(maxlen=100/minlen=34); img=-width,-alt';
    $processed = htmLawed($text, $config, $spec);

  Or,

    $processed = htmLawed($text, $config, 'i=-*; td, tr=style, id, -*; a=id(match="/[a-z][a-z\d.:\-`"]*/i"/minval=2), href(maxlen=100/minlen=34); img=-width,-alt');

  A rule begins with an HTML element name(s) (rule-element), for which the rule applies, followed by an equal (=) sign. A rule-element may represent multiple elements if comma (,)-separated element names are used. E.g., th,td,tr=.

  Rest of the rule consists of comma-separated HTML attribute names. A minus (-) character before an attribute means that the attribute is not permitted inside the rule-element. E.g., -width. To deny all attributes, -* can be used.

  Following shows examples of rule excerpts with rule-element a and the attributes that are being permitted:

  *  a= - all
  *  a=id - all
  *  a=href, title, -id, -onclick - all except id and onclick
  *  a=*, id, -id - all except id
  *  a=-* - none
  *  a=-*, href, title - none except href and title
  *  a=-*, -id, href, title - none except href and title

  Rules regarding attribute values are optionally specified inside round brackets after attribute names in slash ('/')-separated parameter = value pairs. E.g., title(maxlen=30/minlen=5). None, or one or more of the following parameters may be specified:

  *  oneof - one or more choices separated by | that the value should match; if only one choice is provided, then the value must match that choice

  *  noneof - one or more choices separated by | that the value should not match

  *  maxlen and minlen - upper and lower limits for the number of characters in the attribute value; specified in numbers

  *  maxval and minval - upper and lower limits for the numerical value specified in the attribute value; specified in numbers

  *  match and nomatch - pattern that the attribute value should or should not match; specified as PHP/PCRE-compatible regular expressions with delimiters and possibly modifiers

  *  default - a value to force on the attribute if the value provided by the writer does not fit any of the specified parameters

  If default is not set and the attribute value does not satisfy any of the specified parameters, then the attribute is removed. The default value can also be used to force all attribute declarations to take the same value (by getting the values declared illegal by setting, e.g., maxlen to -1).

  Examples with input <input title="WIDTH" value="10em" /><input title="length" value="5" /> are shown below.

  Rule: input=title(maxlen=60/minlen=6), value
  Output: <input value="10em" /><input title="length" value="5" />

  Rule: input=title(), value(maxval=8/default=6)
  Output: <input title="WIDTH" value="6" /><input title="length" value="5" />

  Rule: input=title(nomatch=$w.d$i), value(match=$em$/default=6em)
  Output: <input value="10em" /><input title="length" value="6em" />

  Rule: input=title(oneof=height|depth/default=depth), value(noneof=5|6)
  Output: <input title="depth" value="10em" /><input title="depth" />

  Special characters: The characters ;, ,, /, (, ), |, ~ and space have special meanings in the rules. Words in the rules that use such characters, or the characters themselves, should be escaped by enclosing in pairs of double-quotes ("). A back-tick (`) can be used to escape a literal ". An example rule illustrating this is input=value(maxlen=30/match="/^\w/"/default="your `"ID`"").

  Note: To deny an attribute for all elements for which it is legal, $config["deny_attribute"] can be used instead of $spec. The hook_tag parameter (section 3.4.9) can also be used to implement the $spec functionality.

2.4  Performance time & memory usage

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  The time and memory used by htmLawed depends on its configuration and the size of the input, and the amount, nestedness and well-formedness of the HTML markup within it. In particular, tag balancing and beautification each can increase the processing time by about a quarter.

  The htmLawed demo can be used to evaluate the performance and effects of different types of input and $config.

2.5  Some security risks to keep in mind

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  When setting the parameters/arguments (like those to allow certain HTML elements) for use with htmLawed, potentially dangerous code may get through. (This may not be a problem if the authors are trusted.)

  For example, following increase security risks:

  *  Allowing script, applet, embed, iframe or object elements, or certain of their attributes like allowscriptaccess

  *  Allowing HTML comments (some Internet Explorer versions are vulnerable with, e.g., <!--[if gte IE 4]><script>alert("xss");</script><![endif]-->

  Unsafe HTML can be removed by setting $config appropriately. E.g., $config["elements"] = "* -script" (section 3.3), $config["safe"] = 1 (section 3.6), etc.

2.6  Use without modifying old kses() code

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  The Kses PHP script is used by many applications (like WordPress). It is possible to have such applications use htmLawed instead, since it is compatible with code that calls the kses() function declared in the Kses file (usually named kses.php). E.g., application code like this will continue to work after replacing Kses with htmLawed:

    $comment_filtered = kses($comment_input, array('a'=>array(), 'b'=>array(), 'i'=>array()));

  For some of the $config parameters, htmLawed will use values other than the default ones. These are indicated by ^ in section 2.2. To force htmLawed to use other values, function kses() in the htmLawed code should be edited -- a few configurable parameters/variables need to be changed.

  If the application uses a Kses file that has the kses() function declared, then, to have the application use htmLawed instead of Kses, simply rename htmLawed.php (to kses.php, e.g.) and replace the Kses file (or just replace the code in the Kses file with the htmLawed code). If the kses() function in the Kses file had been renamed by the application developer (e.g., in WordPress, it is named wp_kses()), then appropriately rename the kses() function in the htmLawed code.

  If the Kses file used by the application has been highly altered by the application developers, then one may need a different approach. E.g., with WordPress, it is best to copy the htmLawed code to wp_includes/kses.php, rename the newly added function kses() to wp_kses(), and delete the code for the original wp_kses() function.

  If the Kses code has a non-empty hook function (e.g., wp_kses_hook() in case of WordPress), then the code for htmLawed's kses_hook() function should be appropriately edited. However, the requirement of the hook function should be re-evaluated considering that htmLawed has extra capabilities. With WordPress, the hook function is an essential one. The following code is suggested for the htmLawed kses_hook() in case of WordPress:

    function kses_hook($string, &$cf, &$spec){
    // kses compatibility
    $allowed_html = $spec;
    $allowed_protocols = array();
    foreach($cf['schemes'] as $v){
     foreach($v as $k2=>$v2){
      if(!in_array($k2, $allowed_protocols)){
       $allowed_protocols[] = $k2;
      }
     }
    }
    return wp_kses_hook($string, $allowed_html, $allowed_protocols);
    // eof
    }

2.7  Tolerance for ill-written HTML

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  htmLawed can work with ill-written HTML code in the input. However, HTML that is too ill-written may not be read as HTML, and be considered mere plain text instead. Following statements indicate the degree of looseness that htmLawed can work with, and can be provided in instructions to writers:

  *  Tags must be flanked by < and > with no > inside -- any needed > should be put in as &gt;. It is possible for tag content (element name and attributes) to be spread over many lines instead of being on one. A space may be present between the tag content and >, like <div > and <img / >, but not after the <.

  *  Element and attribute names need not be lower-cased.

  *  Attribute string of elements may be liberally spaced with tabs, line-breaks, etc.

  *  Attribute values may not be double-quoted, or may be single-quoted.

  *  Left-padding of numeric entities (like, &#0160;, &x07ff;) with 0 is okay as long as the number of characters between between the & and the ; does not exceed 8. All entities must end with ; though.

  *  Named character entities must be properly cased. E.g., &Lt; or &TILDE; will not be let through without modification.

  *  HTML comments should not be inside element tags (okay between tags), and should begin with <!-- and end with -->. Characters like <, >, and & may be allowed inside depending on $config, but any --> inside should be put in as --&gt;. Any -- inside will be automatically converted to -, and a space will be added before the comment delimiter -->.

  *  CDATA sections should not be inside element tags, and can be in element content only if plain text is allowed for that element. They should begin with <[CDATA[ and end with ]]>. Characters like <, >, and & may be allowed inside depending on $config, but any ]]> inside should be put in as ]]&gt;.

  *  For attribute values, character entities &lt;, &gt; and &amp; should be used instead of characters < and >, and & (when & is not part of a character entity). This applies even for Javascript code in values of attributes like onclick.

  *  Characters <, >, & and " that are part of actual Javascript, etc., code in script elements should be used as such and not be put in as entities like &gt;. Otherwise, though the HTML will be valid, the code may fail to work. Further, if such characters have to be used, then they should be put inside CDATA sections.

  *  Simple instructions like "an opening tag cannot be present between two closing tags" and "nested elements should be closed in the reverse order of how they were opened" can help authors write balanced HTML. If tags are imbalanced, htmLawed will try to balance them, but in the process, depending on $config["keep_bad"], some code/text may be lost.

  *  Input authors should be notified of admin-specified allowed elements, attributes, configuration values (like conversion of named entities to numeric ones), etc.

  *  With $config["unique_ids"] not 0 and the id attribute being permitted, writers should carefully avoid using duplicate or invalid id values as even though htmLawed will correct/remove the values, the final output may not be the one desired. E.g., when <a id="home"></a><input id="home" /><label for="home"></label> is processed into
<a id="home"></a><input id="prefix_home" /><label for="home"></label>.

  *  Note that even if intended HTML is lost in a highly ill-written input, the processed output will be more secure and standard-compliant.

  *  For URLs, unless $config["scheme"] is appropriately set, writers should avoid using escape characters or entities in schemes. E.g., htt&#112; (which many browsers will read as the harmless http) may be considered bad by htmLawed.

  * htmLawed will attempt to put plain text present directly inside blockquote, form, map and noscript elements (illegal as per the specs) inside auto-generated div elements.

2.8  Limitations & work-arounds

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  htmLawed's main objective is to make the input text more standard-compliant, secure for web-page readers, and free of HTML elements and attributes considered undesirable by the administrator. Some of its current limitations, regardless of this objective, are noted below along with work-arounds.

  It should be borne in mind that no browser application is 100% standard-compliant, and that some of the standard specs (like asking for normalization of white-spacing within textarea elements) are clearly wrong. Regarding security, note that unsafe HTML code is not necessarily legally invalid.

  *  htmLawed is meant for input that goes into the body of HTML documents. HTML's head-level elements are not supported, nor are the frameset elements frameset, frame and noframes.

  *  It cannot transform the non-standard embed elements to the standard-compliant object elements. Yet, it can allow embed elements if permitted (embed is widely used and supported). Admins can certainly use the hook_tag parameter (section 3.4.9) to deploy a custom embed-to-object converter function.

  *  The only non-standard element that may be permitted is embed; others like noembed and nobr cannot be permitted without modifying the htmLawed code.

  *  It cannot handle input that has non-HTML code like SVG and MathML. One way around is to break the input into pieces and passing only those without non-HTML code to htmLawed. Another is described in section 3.9. A third way may be to some how take advantage of the $config["and_mark"] parameter (see section 3.2).

  *  By default, htmLawed won't check many attribute values for standard compliance. E.g., width="20m" with the dimension in non-standard m is let through. Implementing universal and strict attribute value checks can make htmLawed slow and resource-intensive. Admins should look at the hook_tag parameter (section 3.4.9) or $spec to enforce finer checks.

  *  The attributes, deprecated (which can be transformed too) or not, that it supports are largely those that are in the specs. Only a few of the proprietary attributes are supported.

  *  Except for contained URLs and dynamic expressions (also optional), htmLawed does not check CSS style property values. Admins should look at using the hook_tag parameter (section 3.4.9) or $spec for finer checks. Perhaps the best option is to disallow style but allow class attributes with the right oneof or match values for class, and have the various class style properties in .css CSS stylesheet files.

  *  htmLawed does not parse emoticons, decode BBcode, or wikify, auto-converting text to proper HTML. Similarly, it won't convert line-breaks to br elements. Such functions are beyond its purview. Admins should use other code to pre- or post-process the input for such purposes.

  *  htmLawed cannot be used to have links force-opened in new windows (by auto-adding appropriate target and onclick attributes to a). Admins should look at Javascript-based DOM-modifying solutions for this. Admins may also be able to use a custom hook function to enforce such checks (hook_tag parameter; see section 3.4.9).

  *  Nesting-based checks are not possible. E.g., one cannot disallow p elements specifically inside td while permitting it elsewhere. Admins may be able to use a custom hook function to enforce such checks (hook_tag parameter; see section 3.4.9).

  *  Except for optionally converting absolute or relative URLs to the other type, htmLawed will not alter URLs (e.g., to change the value of query strings or to convert http to https. Having absolute URLs may be a standard-requirement, e.g., when HTML is embedded in email messages, whereas altering URLs for other purposes is beyond htmLawed's goals. Admins may be able to use a custom hook function to enforce such checks (hook_tag parameter; see section 3.4.9).

  *  Pairs of opening and closing tags that do not enclose any content (like <em></em>) are not removed. This may be against the standard specs for certain elements (e.g., table). However, presence of such standard-incompliant code will not break the display or layout of content. Admins can also use simple regex-based code to filter out such code.

  *  htmLawed does not check for certain element orderings described in the standard specs (e.g., in a table, tbody is allowed before tfoot). Admins may be able to use a custom hook function to enforce such checks (hook_tag parameter; see section 3.4.9).

  *  htmLawed does not check the number of nested elements. E.g., it will allow two caption elements in a table element, illegal as per the specs. Admins may be able to use a custom hook function to enforce such checks (hook_tag parameter; see section 3.4.9).

  *  htmLawed does not correct certain possible attribute-based security vulnerabilities (e.g., <a href="http://x%22+style=%22background-image:xss">x</a>). Theses arise when browsers mis-identify markup in escaped text, defeating the very purpose of escaping text (a bad browser will read the given example as <a href="http://x" style="background-image:xss">x</a>).

  *  Because of poor Unicode support in PHP, htmLawed does not remove the high value HTML-invalid characters with multi-byte code-points. Such characters however are extremely unlikely to be in the input. (see section 3.1).

  *  Like any script using PHP's PCRE regex functions, PHP setup-specific low PCRE limit values can cause htmLawed to at least partially fail with very long input texts.

2.9  Examples

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  1. A blog administrator wants to allow only a, em, strike, strong and u in comments, but needs strike and u transformed to span for better XHTML 1-strict compliance, and, he wants the a links to be to http or https resources:

    $processed = htmLawed($in, array('elements'=>'a, em, strike, strong, u', 'make_tag_strict'=>1, 'safe'=>1, 'schemes'=>'*:http, https'), 'a=href');

  2. An author uses a custom-made web application to load content on his web-site. He is the only one using that application and the content he generates has all types of HTML, including scripts. The web application uses htmLawed primarily as a tool to correct errors that creep in while writing HTML and to take care of the occasional bad characters in copy-paste text introduced by Microsoft Office. The web application provides a preview before submitted input is added to the content. For the previewing process, htmLawed is set up as follows:

    $processed = htmLawed($in, array('css_expression'=>1, 'keep_bad'=>1, 'make_tag_strict'=>1, 'schemes'=>'*:*', 'valid_xhtml'=>1));

  For the final submission process, keep_bad is set to 6. A value of 1 for the preview process allows the author to note and correct any HTML mistake without losing any of the typed text.

  3. A data-miner is scraping information in a specific table of similar web-pages and is collating the data rows, and uses htmLawed to reduce unnecessary markup and white-spaces:

    $processed = htmLawed($in, array('elements'=>'tr, td', 'tidy'=>-1), 'tr, td =');

3.1  Invalid/dangerous characters

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  Valid characters (more correctly, their code-points) in HTML or XML are, hexadecimally, 9, a, d, 20 to d7ff, and e000 to 10ffff, except fffe and ffff (decimally, 9, 10, 13, 32 to 55295, and 57344 to 1114111, except 65534 and 65535). htmLawed removes the invalid characters 0 to 8, b, c, and e to 1f.

  Because of PHP's poor native support for multi-byte characters, htmLawed cannot check for the remaining invalid code-points. However, for various reasons, it is very unlikely for any of those characters to be in the input.

  Characters that are discouraged (see section 5.1) but not invalid are not removed by htmLawed.

  It (function hl_tag()) also replaces the potentially dangerous (in some Mozilla [Firefox] and Opera browsers) soft-hyphen character (code-point, hexadecimally, ad, or decimally, 173) in attribute values with spaces. Where required, the characters <, >, &, and " are converted to entities.

  With $config["clean_ms_char"] set as 1 or 2, many of the discouraged characters (decimal code-points 127 to 159 except 133) that many Microsoft applications incorrectly use (as per the Windows 1252 [Cp-1252] or a similar encoding system), and the character for decimal code-point 133, are converted to appropriate decimal numerical entities (or removed for a few cases)-- see appendix in section 5.4. This can help avoid some display issues arising from copying-pasting of content.

  With $config["clean_ms_char"] set as 2, characters for the hexadecimal code-points 82, 91, and 92 (for special single-quotes), and 84, 93, and 94 (for special double-quotes) are converted to ordinary single and double quotes respectively and not to entities.

  The character values are replaced with entities/characters and not character values referred to by the entities/characters to keep this task independent of the character-encoding of input text.

  The $config["clean_ms_char"] parameter need not be used if authors do not copy-paste Microsoft-created text or if the input text is not believed to use the Windows 1252 or a similar encoding. Further, the input form and the web-pages displaying it or its content should have the character encoding appropriately marked-up.

3.2  Character references/entities

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  Valid character entities take the form &*; where * is #x followed by a hexadecimal number (hexadecimal numeric entity; like &#xA0; for non-breaking space), or alphanumeric like gt (external or named entity; like &nbsp; for non-breaking space), or # followed by a number (decimal numeric entity; like &#160; for non-breaking space). Character entities referring to the soft-hyphen character (the &shy; or \xad character; hexadecimal code-point ad [decimal 173]) in attribute values are always replaced with spaces; soft-hyphens in attribute values introduce vulnerabilities in some older versions of the Opera and Mozilla [Firefox] browsers.

  htmLawed (function hl_ent()):

  *  Neutralizes entities with multiple leading zeroes or missing semi-colons (potentially dangerous)

  *  Lowercases the X (for XML-compliance) and A-F of hexadecimal numeric entities

  *  Neutralizes entities referring to characters that are HTML-invalid (see section 3.1)

  *  Neutralizes entities referring to characters that are HTML-discouraged (code-points, hexadecimally, 7f to 84, 86 to 9f, and fdd0 to fddf, or decimally, 127 to 132, 134 to 159, and 64991 to 64976). Entities referring to the remaining discouraged characters (see section 5.1 for a full list) are let through.

  *  Neutralizes named entities that are not in the specs.

  *  Optionally converts valid HTML-specific named entities except &gt;, &lt;, &quot;, and &amp; to decimal numeric ones (hexadecimal if $config["hexdec_entity"] is 2) for generic XML-compliance. For this, $config["named_entity"] should be 1.

  *  Optionally converts hexadecimal numeric entities to the more widely supported decimal ones. For this, $config["hexdec_entity"] should be 0.

  *  Optionally converts decimal numeric entities to the hexadecimal ones. For this, $config["hexdec_entity"] should be 2.

  Neutralization refers to the entitification of & to &amp;.

  Note: htmLawed does not convert entities to the actual characters represented by them; one can pass the htmLawed output through PHP's html_entity_decode function for that.

  Note: If $config["and_mark"] is set, and set to a value other than 0, then the & characters in the original input are replaced with the control character for the hexadecimal code-point 6 (\x06; & characters introduced by htmLawed, e.g., after converting < to &lt;, are not affected). This allows one to distinguish, say, an &gt; introduced by htmLawed and an &gt; put in by the input writer, and can be helpful in further processing of the htmLawed-processed text (e.g., to identify the character sequence o(><)o to generate an emoticon image). When this feature is active, admins should ensure that the htmLawed output is not directly used in web pages or XML documents as the presence of the \x06 can break documents. Before use in such documents, and preferably before any storage, any remaining \x06 should be changed back to &, e.g., with:

    $final = str_replace("\x06", '&', $prelim);

  Also, see section 3.9.

3.3  HTML elements

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  htmLawed can be configured to allow only certain HTML elements (tags) in the input. Disallowed elements (just tag-content, and not element-content), based on $config["keep_bad"], are either neutralized (converted to plain text by entitification of < and >) or removed.

  E.g., with only em permitted:

  Input:

      <em>My</em> website is <a href="http://a.com>a.com</a>.

  Output, with $config["keep_bad"] = 0:

      <em>My</em> website is a.com.

  Output, with $config["keep_bad"] not 0:

      <em>My</em> website is &lt;a href=""&gt;a.com&lt;/a&gt;.

  See section 3.3.3 for differences between the various non-zero $config["keep_bad"] values.

  htmLawed by default permits these 86 elements:

    a, abbr, acronym, address, applet, area, b, bdo, big, blockquote, br, button, caption, center, cite, code, col, colgroup, dd, del, dfn, dir, div, dl, dt, em, embed, fieldset, font, form, h1, h2, h3, h4, h5, h6, hr, i, iframe, img, input, ins, isindex, kbd, label, legend, li, map, menu, noscript, object, ol, optgroup, option, p, param, pre, q, rb, rbc, rp, rt, rtc, ruby, s, samp, script, select, small, span, strike, strong, sub, sup, table, tbody, td, textarea, tfoot, th, thead, tr, tt, u, ul, var

  Except for embed (included because of its wide-spread use) and the Ruby elements (rb, rbc, rp, rt, rtc, ruby; part of XHTML 1.1), these are all the elements in the HTML 4/XHTML 1 specs. Strict-specific specs. exclude center, dir, font, isindex, menu, s, strike, and u.

  With $config["safe"] = 1, the default set will exclude applet, embed, iframe, object and script; see section 3.6.

  When $config["elements"], which specifies allowed elements, is properly defined, and neither empty nor set to 0 or *, the default set is not used. To have elements added to or removed from the default set, a +/- notation is used. E.g., *-script-object implies that only script and object are disallowed, whereas *+embed means that noembed is also allowed. Elements can also be specified as comma separated names. E.g., a, b, i means only a, b and i are permitted. In this notation, *, + and - have no significance and can actually cause a mis-reading.

  Some more examples of $config["elements"] values indicating permitted elements (note that empty spaces are liberally allowed for clarity):

  *  a, blockquote, code, em, strong -- only a, blockquote, code, em, and strong
  *  *-script -- all excluding script
  *  * -center -dir -font -isindex -menu -s -strike -u -- only XHTML-Strict elements
  *  *+noembed-script -- all including noembed excluding script

  Some mis-usages (and the resulting permitted elements) that can be avoided:

  *  -* -- none; one should rather use, e.g., the htmlspecialchars() PHP function on the input than htmLawed
  *  *, -script -- all except script; admin probably meant *-script
  *  -*, a, em, strong -- all; admin probably meant a, em, strong
  *  * -- all; admin need not have set elements
  *  *-form+form -- all; a + will always over-ride any -
  *  *, noembed -- only noembed; admin probably meant *+noembed
  *  a, +b, i -- only a and i; admin probably meant a, b, i

  Basically, when using the +/- notation, commas (,) should not be used, and vice versa, and * should be used with the former but not the latter.

  Note: Even if an element that is not in the default set is allowed through $config["elements"], like noembed in the last example, it will eventually be removed during tag balancing unless such balancing is turned off ($config["balance"] set to 0). Currently, the only way around this, which actually is simple, is to edit the various arrays in the function hl_bal() to accommodate the element and its nesting properties.

  A possibly second way to specify allowed elements is to set $config["parent"] to an element name that supposedly will hold the input, and to set $config["balance"] to 1. During tag balancing (see section 3.3.3), all elements that cannot legally nest inside the parent element will be removed. The parent element is auto-reset to div if $config["parent"] is empty, body, or an element not in htmLawed's default set of 86 elements.

  Tag transformation is possible for improving XHTML-Strict compliance -- most of the deprecated elements are removed or converted to valid XHTML-Strict ones; see section 3.3.2.

3.4  Attributes

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  htmLawed will only permit attributes described in the HTML specs (including deprecated ones). It also permits some attributes for use with the embed element (the non-standard embed element is supported in htmLawed because of its widespread use), and the the xml:space attribute (valid only in XHTML 1.1). A list of such 111 attributes and the elements they are allowed in is in section 5.2.

  When $config["deny_attribute"] is not set, or set to 0, or empty (""), all the 111 attributes are permitted. Otherwise, $config["deny_attribute"] can be set as a list of comma-separated names of the denied attributes. on* can be used to refer to the group of potentially dangerous, script-accepting attributes: onblur, onchange, onclick, ondblclick, onfocus, onkeydown, onkeypress, onkeyup, onmousedown, onmousemove, onmouseout, onmouseover, onmouseup, onreset, onselect and onsubmit.

  With $config["safe"] = 1, the on* attributes are automatically disallowed.

  htmLawed (function hl_tag()) also:

  *  Lower-cases attribute names
  *  Removes duplicate attributes (last one stays)
  *  Gives attributes the form name="value" and single-spaces them, removing unnecessary white-spacing
  *  Provides required attributes (see section 3.4.1)
  *  Double-quotes values and escapes any " inside them
  *  Replaces the possibly dangerous soft-hyphen characters (hexadecimal code-point ad) in the values with spaces
  *  Allows custom function to additionally filter/modify attribute values (see section 3.4.9)

3.5  Simple configuration directive for most valid XHTML

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  If $config["valid_xhtml"] is set to 1, some relevant $config parameters (indicated by ~ in section 2.2) are auto-adjusted. This allows one to pass the $config argument with a simpler value. If a value for a parameter auto-set through valid_xhtml is still manually provided, then that value will over-ride the auto-set value.

3.6  Simple configuration directive for most safe HTML

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  Safe HTML refers to HTML that is restricted to reduce the vulnerability for scripting attacks (such as XSS) based on HTML code which otherwise may still be legal and compliant with the HTML standard specs. When elements such as script and object, and attributes such as onmouseover and style are allowed in the input text, an input writer can introduce malevolent HTML code. Note that what is considered safe depends on the nature of the web application and the trust-level accorded to its users.

  htmLawed allows an admin to use $config["safe"] to auto-adjust multiple $config parameters (such as elements which declares the allowed element-set), which otherwise would have to be manually set. The relevant parameters are indicated by " in section 2.2). Thus, one can pass the $config argument with a simpler value.

  With the value of 1, htmLawed considers CDATA sections and HTML comments as plain text, and prohibits the applet, embed, iframe, object and script elements, and the on* attributes like onclick. Further, URLs with schemes (see section 3.4.3) are neutralized so that, e.g., style="moz-binding:url(http://danger)" becomes style="moz-binding:url(denied:http://danger)" while style="moz-binding:url(ok)" remains intact.

  Admins, however, may still want to completely deny the style attribute, e.g., with code like

    $processed = htmLawed($text, array('safe'=>1, 'deny_attribute'=>'on*, style'));

  There are $config parameters like css_expression that are not affected by the value set for safe but whose default values still contribute towards a more safe output.

  If a value for a parameter auto-set through safe is still manually provided, then that value can over-ride the auto-set value. E.g., with $config["safe"] = 1 and $config["elements"] = "*+script", script, but not applet, is allowed.

  A page illustrating the efficacy of htmLawed's anti-XSS abilities with safe set to 1 against XSS vectors listed by RSnake may be available here.

3.7  Using a hook function

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  If $config["hook"] is not set to 0, then htmLawed will allow preliminarily processed input to be altered by a hook function named by $config["hook"] before starting the main work (but after handling of characters, entities, HTML comments and CDATA sections -- see code for function htmLawed()).

  The hook function also allows one to alter the finalized values of $config and $spec.

  Note that the hook parameter is different from the hook_tag parameter (section 3.4.9).

  Snippets of hook function code developed by others may be available on the htmLawed website.

3.8  Obtaining finalized parameter values

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  htmLawed can assign the finalized $config and $spec values to a variable named by $config["show_setting"]. The variable, made global by htmLawed, is set as an array with three keys: config, with the $config value, spec, with the $spec value, and time, with a value that is the Unix time (the output of PHP's microtime() function) when the value was assigned. Admins should use a PHP-compliant variable name (e.g., one that does not begin with a numerical digit) that does not conflict with variable names in their non-htmLawed code.

  The values, which are also post-hook function (if any), can be used to auto-generate information (on, e.g., the elements that are permitted) for input writers.

3.9  Retaining non-HTML tags in input with mixed markup

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  htmLawed does not remove certain characters that though invalid are nevertheless discouraged in HTML documents as per the specs (see section 5.1). This can be utilized to deal with input that contains mixed markup. Input that may have HTML markup as well as some other markup that is based on the <, > and & characters is considered to have mixed markup. The non-HTML markup can be rather proprietary (like markup for emoticons/smileys), or standard (like MathML or SVG). Or it can be programming code meant for execution/evaluation (such as embedded PHP code).

  To deal with such mixed markup, the input text can be pre-processed to hide the non-HTML markup by specifically replacing the <, > and & characters with some of the HTML-discouraged characters (see section 3.1.2). Post-htmLawed processing, the replacements are reverted.

  An example (mixed HTML and PHP code in input text):

    $text = preg_replace('`<\?php(.+?)\?>`sm', "\x83?php\\1?\x84", $text);
    $processed = htmLawed($text);
    $processed = preg_replace('`\x83\?php(.+?)\?\x84`sm', '<?php$1?>', $processed);

  This code will not work if $config["clean_ms_char"] is set to 1 (section 3.1), in which case one should instead deploy a hook function (section 3.7). (htmLawed internally uses certain control characters, code-points 1 to 7, and use of these characters as markers in the logic of hook functions may cause issues.)

  Admins may also be able to use $config["and_mark"] to deal with such mixed markup; see section 3.2.

4.1  Support

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  A careful re-reading of this documentation will very likely answer your questions.

  Software updates and forum-based community-support may be found at http://www.bioinformatics.org/phplabware/internal_utilities/htmLawed. For general PHP issues (not htmLawed-specific), support may be found through internet searches and at http://php.net.

4.2  Known issues

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  See section 2.8.

  Readers are advised to cross-check information given in this document.

4.3  Change-log

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  (The release date for the downloadable package of files containing documentation, demo script, test-cases, etc., besides the htmLawed.php file may be updated independently if the secondary files are revised.)

  Key: Version number - Release date. Notes

  1.1 - 29 June 2008. $config["hook_tag"] and $config["format"] introduced for custom tag/attribute check/modification/injection and output compaction/beautification; fixed a regex-in-$spec parsing bug

  1.0.9 - 11 June 2008. Fixed bug in invalid HTML code-point entity check

  1.0.8 - 15 May 2008. bordercolor attribute for table, td and tr

  1.0.7 - 1 May 2008. Support for wmode attribute for embed; $config["show_setting"] introduced; improved $config["elements"] evaluation

  1.0.6 - 20 April 2008. $config["and_mark"] introduced

  1.0.5 - 12 March 2008. style URL schemes essentially disallowed when $config safe is on; improved regex for CSS expression search

  1.0.4 - 10 March 2008. Improved corrections for blockquote, form, map and noscript

  1.0.3 - 3 March 2008. Character entities for soft-hyphens are now replaced with spaces (instead of being removed); a bug allowing td directly inside table fixed; safe $config parameter added

  1.0.2 - 13 February 2008. Improved implementation of $config["keep_bad"]

  1.0.1 - 7 November 2007. Improved regex for identifying URLs, protocols and dynamic expressions (hl_tag() and hl_prot()); no error display with hl_regex()

  1.0 - 2 November 2007. First release

4.4  Testing

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  To test htmLawed using a form interface, a demo web-page is provided with the htmLawed distribution (htmLawed.php and htmLawedTest.php should be in the same directory on the web-server). A file with test-cases is also provided.

4.5  Upgrade, & old versions

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  Upgrading is as simple as replacing the previous version of htmLawed.php (assuming it was not modified for customized features). As htmLawed output is almost always used in static documents, upgrading should not affect old, finalized content.

  Old versions of htmLawed may be available online. E.g., for version 1.0, check http://www.bioinformatics.org/phplabware/downloads/htmLawed1.zip, for 1.1.1, htmLawed111.zip, and for 1.1.10, htmLawed1110.zip.

4.6  Comparison with HTMLPurifier

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  The HTMLPurifier PHP library by Edward Yang is a very good HTML filtering script that uses object oriented PHP code. Compared to htmLawed, it:

  *  does not support PHP versions older than 5.0 (HTMLPurifier dropped PHP 4 support after version 2)

  *  is 15-20 times bigger (scores of files totalling more than 750 kb)

  *  consumes 10-15 times more RAM memory (just including the HTMLPurifier files without calling the filter requires a few MBs of memory)

  *  is expectedly slower

  *  does not allow admins to fully allow all valid HTML (because of incomplete HTML support, it always considers elements like script illegal)

  *  lacks many of the extra features of htmLawed (like entity conversions and code compaction/beautification)

  *  has poor documentation

  However, HTMLPurifier has finer checks for character encodings and attribute values, and can log warnings and errors. Visit the HTMLPurifier website for updated information.

4.7  Use through application plug-ins/modules

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  Plug-ins/modules to implement htmLawed in applications such as Drupal and DokuWiki may have been developed. Please check the application websites and the forum on the htmLawed site.

4.8  Use in non-PHP applications

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  Non-PHP applications written in Python, Ruby, etc., may be able to use htmLawed through system calls to the PHP engine. Such code may have been documented on the internet. Also check the forum on the htmLawed site.

4.9  Donate

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  A donation in any currency and amount to appreciate or support this software can be sent by PayPal to this email address: drpatnaik at yahoo dot com.

4.10  Acknowledgements

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  Ulf Harnhammer, Lukasz Pilorz, Shelley Powers, Edward Yang, and many anonymous users.

  Thank you!

5.1  Characters discouraged in XHTML

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  Characters represented by the following hexadecimal code-points are not invalid, even though some validators may issue messages stating otherwise.

  7f to 84, 86 to 9f, fdd0 to fddf, 1fffe, 1ffff, 2fffe, 2ffff, 3fffe, 3ffff, 4fffe, 4ffff, 5fffe, 5ffff, 6fffe, 6ffff, 7fffe, 7ffff, 8fffe, 8ffff, 9fffe, 9ffff, afffe, affff, bfffe, bffff, cfffe, cffff, dfffe, dffff, efffe, effff, ffffe, fffff, 10fffe and 10ffff

5.2  Valid attribute-element combinations

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  Valid attribute-element combinations as per W3C specs.

  *  includes deprecated attributes (marked ^), attributes for the non-standard embed element (marked *), and the proprietary bordercolor (marked ~)
  *  only non-frameset, HTML body elements
  *  name for a and map, and lang are invalid in XHTML 1.1
  *  target is valid for a in XHTML 1.1 and higher
  *  xml:space is only for XHTML 1.1

  abbr - td, th
  accept - form, input
  accept-charset - form
  accesskey - a, area, button, input, label, legend, textarea
  action - form
  align - caption^, embed, applet, iframe, img^, input^, object^, legend^, table^, hr^, div^, h1^, h2^, h3^, h4^, h5^, h6^, p^, col, colgroup, tbody, td, tfoot, th, thead, tr
  alt - applet, area, img, input
  archive - applet, object
  axis - td, th
  bgcolor - embed, table^, tr^, td^, th^
  border - table, img^, object^
  bordercolor~ - table, td, tr
  cellpadding - table
  cellspacing - table
  char - col, colgroup, tbody, td, tfoot, th, thead, tr
  charoff - col, colgroup, tbody, td, tfoot, th, thead, tr
  charset - a, script
  checked - input
  cite - blockquote, q, del, ins
  classid - object
  clear - br^
  code - applet
  codebase - object, applet
  codetype - object
  color - font
  cols - textarea
  colspan - td, th
  compact - dir, dl^, menu, ol^, ul^
  coords - area, a
  data - object
  datetime - del, ins
  declare - object
  defer - script
  dir - bdo
  disabled - button, input, optgroup, option, select, textarea
  enctype - form
  face - font
  for - label
  frame - table
  frameborder - iframe
  headers - td, th
  height - embed, iframe, td^, th^, img, object, applet
  href - a, area
  hreflang - a
  hspace - applet, img^, object^
  ismap - img, input
  label - option, optgroup
  language - script^
  longdesc - img, iframe
  marginheight - iframe
  marginwidth - iframe
  maxlength - input
  method - form
  model* - embed
  multiple - select
  name - button, embed, textarea, applet^, select, form^, iframe^, img^, a^, input, object, map^, param
  nohref - area
  noshade - hr^
  nowrap - td^, th^
  object - applet
  onblur - a, area, button, input, label, select, textarea
  onchange - input, select, textarea
  onfocus - a, area, button, input, label, select, textarea
  onreset - form
  onselect - input, textarea
  onsubmit - form
  pluginspage* - embed
  pluginurl* - embed
  prompt - isindex
  readonly - textarea, input
  rel - a
  rev - a
  rows - textarea
  rowspan - td, th
  rules - table
  scope - td, th
  scrolling - iframe
  selected - option
  shape - area, a
  size - hr^, font, input, select
  span - col, colgroup
  src - embed, script, input, iframe, img
  standby - object
  start - ol^
  summary - table
  tabindex - a, area, button, input, object, select, textarea
  target - a^, area, form
  type - a, embed, object, param, script, input, li^, ol^, ul^, button
  usemap - img, input, object
  valign - col, colgroup, tbody, td, tfoot, th, thead, tr
  value - input, option, param, button, li^
  valuetype - param
  vspace - applet, img^, object^
  width - embed, hr^, iframe, img, object, table, td^, th^, applet, col, colgroup, pre^
  wmode - embed
  xml:space - pre, script, style

  These are allowed in all but the shown elements:

  class - param, script
  dir - applet, bdo, br, iframe, param, script
  id - script
  lang - applet, br, iframe, param, script
  onclick - applet, bdo, br, font, iframe, isindex, param, script
  ondblclick - applet, bdo, br, font, iframe, isindex, param, script
  onkeydown - applet, bdo, br, font, iframe, isindex, param, script
  onkeypress - applet, bdo, br, font, iframe, isindex, param, script
  onkeyup - applet, bdo, br, font, iframe, isindex, param, script
  onmousedown - applet, bdo, br, font, iframe, isindex, param, script
  onmousemove - applet, bdo, br, font, iframe, isindex, param, script
  onmouseout - applet, bdo, br, font, iframe, isindex, param, script
  onmouseover - applet, bdo, br, font, iframe, isindex, param, script
  onmouseup - applet, bdo, br, font, iframe, isindex, param, script
  style - param, script
  title - param, script
  xml:lang - applet, br, iframe, param, script

5.3  CSS 2.1 properties accepting URLs

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  background
  background-image
  content
  cue-after
  cue-before
  cursor
  list-style
  list-style-image
  play-during

5.4  Microsoft Windows 1252 character replacements

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  Key: d double, l left, q quote, r right, s. single

  Code-point (decimal) - hexadecimal value - replacement entity - represented character

  127 - 7f - (removed) - (not used)
  128 - 80 - &#8364; - euro
  129 - 81 - (removed) - (not used)
  130 - 82 - &#8218; - baseline s. q
  131 - 83 - &#402; - florin
  132 - 84 - &#8222; - baseline d q
  133 - 85 - &#8230; - ellipsis
  134 - 86 - &#8224; - dagger
  135 - 87 - &#8225; - d dagger
  136 - 88 - &#710; - circumflex accent
  137 - 89 - &#8240; - permile
  138 - 8a - &#352; - S Hacek
  139 - 8b - &#8249; - l s. guillemet
  140 - 8c - &#338; - OE ligature
  141 - 8d - (removed) - (not used)
  142 - 8e - &#381; - Z dieresis
  143 - 8f - (removed) - (not used)
  144 - 90 - (removed) - (not used)
  145 - 91 - &#8216; - l s. q
  146 - 92 - &#8217; - r s. q
  147 - 93 - &#8220; - l d q
  148 - 94 - &#8221; - r d q
  149 - 95 - &#8226; - bullet
  150 - 96 - &#8211; - en dash
  151 - 97 - &#8212; - em dash
  152 - 98 - &#732; - tilde accent
  153 - 99 - &#8482; - trademark
  154 - 9a - &#353; - s Hacek
  155 - 9b - &#8250; - r s. guillemet
  156 - 9c - &#339; - oe ligature
  157 - 9d - (removed) - (not used)
  158 - 9e - &#382; - z dieresis
  159 - 9f - &#376; - Y dieresis

5.5  URL format

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  An absolute URL has a protocol or scheme, a network location or hostname, and, optional path, parameters, query and fragment segments. Thus, an absolute URL has this generic structure:

    (scheme) : (//network location) /(path) ;(parameters) ?(query) #(fragment)

  The schemes can only contain letters, digits, +, . and -. Hostname is the portion after the // and up to the first / (if any; else, up to the end) when : is followed by a // (e.g., abc.com in ftp://abc.com/def); otherwise, it consists of everything after the : (e.g., def@abc.com in mailto:def@abc.com').

  Relative URLs do not have explicit schemes and network locations; such values are inherited from a base URL.

5.6  Brief on htmLawed code

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  Much of the code's logic and reasoning can be understood from the documentation above.

  The output of htmLawed is a text string containing the processed input. There is no custom error tracking.

  Function arguments for htmLawed are:

  *  $in - 1st argument; a text string; the input text to be processed. Any extraneous slashes added by PHP when magic quotes are enabled should be removed beforehand using PHP's stripslashes() function.

  *  $config - 2nd argument; an associative array; optional (named $C in htmLawed code). The array has keys with names like balance and keep_bad, and the values, which can be boolean, string, or array, depending on the key, are read to accordingly set the configurable parameters (indicated by the keys). All configurable parameters receive some default value if the value to be used is not specified by the user through $config. Finalized $config is thus a filtered and possibly larger array.

  *  $spec - 3rd argument; a text string; optional. The string has rules, written in an htmLawed-designated format, specifying element-specific attribute and attribute value restrictions. Function hl_spec() is used to convert the string to an associative-array for internal use. Finalized $spec is thus an array.

  Finalized $config and $spec are made global variables while htmLawed is at work. Values of any pre-existing global variables with same names are noted, and their values are restored after htmLawed finishes processing the input (to capture the finalized values, the show_settings parameter of $config should be used). Depending on $config, another global variable hl_Ids, to track id attribute values for uniqueness, may be set. Unlike the other two variables, this one is not reset (or unset) post-processing.

  Except for the main function htmLawed() and the functions kses() and kses_hook(), htmLawed's functions are name-spaced using the hl_ prefix. The functions and their roles are:

  *  hl_attrval - checking attribute values against $spec
  *  hl_bal - tag balancing
  *  hl_cmtcd - handling CDATA sections and HTML comments
  *  hl_ent - entity handling
  *  hl_prot - checking a URL scheme/protocol
  *  hl_regex - checking syntax of a regular expression
  *  hl_spec - converting user-supplied $spec value to one used by htmLawed internally
  *  hl_tag - handling tags
  *  hl_tag2 - transforming tags
  *  hl_tidy - compact/beautify HTML
  *  hl_version - reporting htmLawed version
  *  htmLawed - main function
  *  kses - main function of kses
  *  kses_hook - hook function of kses

  The last two are for compatibility with pre-existing code using the kses script. htmLawed's kses() basically passes on the filtering task to htmLawed() function after deciphering $config and $spec from the argument values supplied to it. kses_hook() is an empty function and is meant for being filled with custom code if the kses script users were using one.

  htmLawed() finalizes $spec (with the help of hl_spec()) and $config, and globalizes them. Finalization of $config involves setting default values if an inappropriate or invalid one is supplied. This includes calling hl_regex() to check well-formedness of regular expression patterns if such expressions are user-supplied through $config. htmLawed() then removes invalid characters like nulls and x01 and appropriately handles entities using hl_ent(). HTML comments and CDATA sections are identified and treated as per $config with the help of hl_cmtcd(). When retained, the < and > characters identifying them, and the <, > and & characters inside them, are replaced with control characters (code-points 1 to 5) till any tag balancing is completed.

  After this initial processing htmLawed() identifies tags using regex and processes them with the help of hl_tag() --  a large function that analyzes tag content, filtering it as per HTML standards, $config and $spec. Among other things, hl_tag() transforms deprecated elements using hl_tag2(), removes attributes from closing tags, checks attribute values as per $spec rules using hl_attrval(), and checks URL protocols using hl_prot(). htmLawed() performs tag balancing and nesting checks with a call to hl_bal(), and optionally compacts/beautifies the output with proper white-spacing with a call to hl_tidy(). The latter temporarily replaces white-space, and <, > and & characters inside pre, script and textarea elements, and HTML comments and CDATA sections with control characters (code-points 1 to 5, and 7).

  htmLawed permits the use of custom code or hook functions at two stages. The first, called inside htmLawed(), allows the input text as well as the finalized $config and $spec values to be altered right after the initial processing (see section 3.7). The second is called by hl_tag() once the tag content is finalized (see section 3.4.9).

  Being dictated by the external and stable HTML standard, htmLawed's objective is very clear-cut and less concerned with tweakability. The code is only minimally annotated with comments -- it is not meant to instruct; PHP developers familiar with the HTML specs will see the logic, and others can always refer to the htmLawed documentation. The compact structuring of the statements is meant to aid in quickly grasping the logic, at least when viewed with code syntax highlighted.