Extension:ParserFunctions/String functions

StringFunctions defines an additional set of parser functions that operate on strings. Version 2.0 resolves inconsistencies with &lt;nowiki> and eliminates the need for PHP's mbstring extension on the installed server.

Note: These functions are currently not installed at the Wikimedia projects.

Functions
This module defines these functions: len, pos, rpos, sub, pad, replace, explode, urlencode, and urldecode</tt>.

All of these functions operate in O(n) time complexity, making them safe against DoS attacks.

Note: Some parameters of these functions are limited through global settings to prevent abuse. See section Limits below.

#len:
The #len function returns the length of the given string. The syntax is:

The return value is always a number of characters in the string. If no string is specified, the return value is zero.

Notes:
 * Trailing spaces are not counted. Example:  returns 8.
 * This function is safe with utf-8 multibyte characters. Example:  returns 8.
 * Tags such as &lt;nowiki> and other tag extensions will always have a length of one, since their content is hidden from the parser. Example:   returns 5.

#pos:
The #pos function returns the position of a given needle within the string. The syntax is:

The offset parameter, if specified, tells a starting position where this function should begin searching.

If the needle is found, the return value is a zero-based integer of the first position within the string. If the needle is not found, the function returns an empty string.

Notes:
 * This function is case sensitive.
 * The maximum allowed length of the needle is limited through the $wgStringFunctionsLimitSearch global setting.
 * This function is safe with utf-8 multibyte characters. Example:  returns 4.
 * As with #len, &lt;nowiki> and other tag extensions are treated as having a length of 1 for the purposes of character position. Example:   returns 1.

#rpos:
The #rpos function returns the last position of a given needle within the string. The syntax is:

If the needle is found, the return value is a zero-based integer of its last position within the string. If the needle is not found, the function returns -1.

Tip: When using this to search for the last delimiter, add +1 to the result to retreive position after the last delimiter. This also works when the delimiter is not found, because "-1 + 1" is zero, which is the beginning of the given value.

Notes:
 * This function is case sensitive.
 * The maximum allowed length of the needle is limited through the $wgStringFunctionsLimitSearch global setting.
 * This function is safe with utf-8 multibyte characters. Example:  returns 4.
 * As with #len, &lt;nowiki> and other tag extensions are treated as having a length of 1 for the purposes of character position. Example:   returns 1.

#sub:
The #sub function returns a substring from the given string. The syntax is:

The start parameter, if positive (or zero), specifies a zero-based index of the first character to be returned. Example:  returns cream</tt>.

If the start parameter is negative, it specifies how many characters from the end should be returned. Example:  returns eam</tt>.

The length parameter, if present and positive, specifies the maximum length of the returned string. Example:  returns cre</tt>.

If the length parameter is negative, it specifies how many characters will be omitted from the end of the string. Example:  returns cr</tt>.

Notes:
 * If the length parameter is zero, it is not used for truncation at all.
 * Example:  returns cream</tt>.
 * If start denotes a position beyond the truncation from the end by negative length parameter, an empty string will be returned.
 * Example:  returns an empty string.
 * This function is safe with utf-8 multibyte characters. Example:  returns žlina</tt>.
 * As with #len, &lt;nowiki> and other tag extensions are treated as having a length of 1 for the purposes of character position. Example:  returns test</tt>.

#pad:
The #pad function returns the given string extended to a given width. The syntax is:

The length parameter specifies the desired length of the returned string.

The padstring parameter, if specified, is used to fill the missing space. It may be a single character, which will be used as many times as necessary; or a string which will be concatenated as many times as necessary and then trimmed to the required length. Example:  returns xXxXxXxIce</tt>.

If the padstring is not specified, spaces are used for padding.

The direction parameter, if specified, can be one of these values:
 * left</tt> - the padding will be on the left side of the string. Example:  returns xxIce</tt>.
 * right</tt> - the padding will be on the right side of the string. Example:  returns Icexx</tt>.
 * center</tt> - the string will be centered in the returned string. Example:  returns xIcex</tt>.

If the direction is not specified, the padding will be on the left side of the string.

The return value is the given string extended to length characters, using the padstring to fill the missing part(s). If the given string is already longer than length, it is neither extended nor truncated.

Notes:
 * The maximum allowed value for the length is limited through the $wgStringFunctionsLimitPad global setting.
 * This function is only partially safe with utf-8 multibyte characters. These characters will be treated appropriately if they appear in the original string, but will not be respected if they appear in the padding. Examples:
 * returns zzzzZmrzlina
 * returns zzzzŽmržlina
 * returns žžŽmržlina
 * Tags such as &lt;nowiki> and other tag extensions are not permitted in the padding. If the padstring contains such a tag, it will be truncated.

#replace:
The #replace function returns the given string with all occurrences of a needle replaced with product.

If the needle is unspecified or empty, a single space will be searched for.

If the product is unspecified or empty, all occurrences of the needle will be removed from the string.

Notes:
 * This function is case sensitive.
 * The maximum allowed length of the needle is limited through the $wgStringFunctionsLimitSearch global setting.
 * The maximum allowed length of the product is limited through the $wgStringFunctionsLimitReplace global setting.
 * Even if the product is a space, an empty string is used. This is a side-effect of the MediaWiki parser. To use a space as the product, put it in nowiki tags.
 * Example:  returns My little home page</tt>.
 * Note that this is the only acceptable use of nowiki in the product, as otherwise nowiki could be used to bypass $wgStringFunctionsLimitReplace, injecting an arbitrarily large number of characters into the output. For this reason, all occurrences of &lt;nowiki> or any other tag extension within the product are replaced with spaces.
 * This function is safe with utf-8 multibyte characters. Example:  returns <tt>Žmrzlina</tt>.

#explode:
The #explode functions splits the given string into pieces and then returns one of the pieces. The syntax is:

The delimiter parameter specifies a string to be used to divide the string into pieces. This delimiter string is then not part of any piece, and when two delimiter strings are next to each other, they create an empty piece between them. If this parameter is not specified, a single space is used.

The position parameter specifies which piece is to be returned. Pieces are counted from 0. If this parameter is not specified, the first piece is used (piece with number 0). When a negative value is used as position, the pieces are counted from the end. In this case, piece number -1 means the last piece. Examples:
 * returns <tt>you</tt>.
 * returns <tt>Code</tt>.

The return value is the position-th piece. If there are less pieces than the position specifies, an empty string is returned.

Notes:
 * This function is case sensitive.
 * The maximum allowed length of the delimiter is limited through $wgStringFunctionsLimitSearch global setting.
 * This function is safe with utf-8 multibyte characters. Example:  returns <tt>lina</tt>.

#urlencode: and #urldecode:
These two functions operate in tandem: #urlencode converts a string into an URL-safe syntax, and #urldecode converts such a string back. The syntax is:

Notes:
 * These functions work by directly exposing PHP's urlencode and urldecode functions.
 * For anchors within a page use instead of  . The results of a call to  are compatible with intra-page references generated with  syntax, while  -generated values are not necessarily so.

Limits
This module defines three global settings: <tt>$wgStringFunctionsLimitSearch</tt>, <tt>$wgStringFunctionsLimitReplace</tt>, <tt>$wgStringFunctionsLimitPad</tt>.

These are used to limit some parameters of some functions to ensure the functions operate in O(n) time complexity, and are therefore safe against DoS attacks.

$wgStringFunctionsLimitSearch
This setting is used by #pos, #rpos, #replace, and #explode. All these functions search for a substring in a larger string while they operate, which can run in O(n*m) and therefore make the software more vulnerable to DoS attacks. By setting this value to a specific small number, the time complexity is decreased to O(n).

This setting limits the maximum allowed length of the string being searched for.

The default value is 30 multibyte characters.

$wgStringFunctionsLimitReplace
This setting is used by #replace. This function replaces all occurrences of one string for another, which can be used to quickly generate very large amounts of data, and therefore makes the software more vulnerable to DoS attacks. This setting limits the maximum allowed length of the replacing string.

The default value is 30 multibyte characters.

$wgStringFunctionsLimitPad
This setting is used by #pad. This function creates a string of the specified length, which can be used to quickly generate very large amounts of data, and therefore makes the software more vulnerable to DoS attacks. This setting limits the maximum allowed length of the resulting padded string.

The default value is 100 multibyte characters.

Installation
This extension requires MediaWiki 1.7+ and PHP 5.

Instructions
Do the following to install these functions as an extension to MediaWiki.
 * 1) Copy the source code below to <tt>extensions/StringFunctions/StringFunctions.php</tt>.
 * 2) Add the following to <tt>LocalSettings.php</tt> (near the bottom) in the root of your MediaWiki installation:

Code
StringFunctions 2.0 has been tested on MediaWiki versions 1.7 and above.

History:
 * Dec 10, 2007 -- v2.0 -- Rewrote the functions to eliminate dependence on mbstring and to properly handle &lt;nowiki> and other tags.
 * Aug 28, 2007 -- v1.10 -- Added negative positions to #explode.
 * Jan 30, 2007 -- v1.9 -- Added limits to #pos, #pad, #replace, #explode.
 * Oct 30, 2006 -- v1.8 -- Fixed for MediaWiki 1.8.
 * Oct 26, 2006 -- v1.6 -- Fixed spaces in #rpos and #replace.
 * Oct 1, 2006 -- v1.5 -- Added #rpos, #pad, #replace, #explode.
 * May 18, 2006 -- v1.2 -- Renamed #toURL and #fromURL to match MediaWiki's &#123;&#123;urlencode:}} function.
 * May 18, 2006 -- v1.1 -- Added #pos.
 * May 15, 2006 -- v1.0 -- First stable release.