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PHP : Function Reference : Regular Expression Functions (Perl-Compatible) : preg_replace

preg_replace

Perform a regular expression search and replace (PHP 4, PHP 5)
mixed preg_replace ( mixed pattern, mixed replacement, mixed subject [, int limit [, int &count]] )

Searches subject for matches to pattern and replaces them with replacement.

Parameters

pattern

The pattern to search for. It can be either a string or an array with strings.

The e modifier makes preg_replace() treat the replacement parameter as PHP code after the appropriate references substitution is done. Tip: make sure that replacement constitutes a valid PHP code string, otherwise PHP will complain about a parse error at the line containing preg_replace().

replacement

The string or an array with strings to replace. If this parameter is a string and the pattern parameter is an array, all patterns will be replaced by that string. If both pattern and replacement parameters are arrays, each pattern will be replaced by the replacement counterpart. If there are fewer elements in the replacement array than in the pattern array, any extra patterns will be replaced by an empty string.

replacement may contain references of the form \\n or (since PHP 4.0.4) $n, with the latter form being the preferred one. Every such reference will be replaced by the text captured by the n'th parenthesized pattern. n can be from 0 to 99, and \\0 or $0 refers to the text matched by the whole pattern. Opening parentheses are counted from left to right (starting from 1) to obtain the number of the capturing subpattern.

When working with a replacement pattern where a backreference is immediately followed by another number (i.e.: placing a literal number immediately after a matched pattern), you cannot use the familiar \\1 notation for your backreference. \\11, for example, would confuse preg_replace() since it does not know whether you want the \\1 backreference followed by a literal 1, or the \\11 backreference followed by nothing. In this case the solution is to use \${1}1. This creates an isolated $1 backreference, leaving the 1 as a literal.

When using the e modifier, this function escapes some characters (namely ', ", \ and NULL) in the strings that replace the backreferences. This is done to ensure that no syntax errors arise from backreference usage with either single or double quotes (e.g. 'strlen(\'$1\')+strlen("$2")'). Make sure you are aware of PHP's string syntax to know exactly how the interpreted string will look like.

subject

The string or an array with strings to search and replace.

If subject is an array, then the search and replace is performed on every entry of subject, and the return value is an array as well.

limit

The maximum possible replacements for each pattern in each subject string. Defaults to -1 (no limit).

count

If specified, this variable will be filled with the number of replacements done.

Return Values

preg_replace() returns an array if the subject parameter is an array, or a string otherwise.

If matches are found, the new subject will be returned, otherwise subject will be returned unchanged.

ChangeLog

Version Description
5.1.0 Added the count parameter
4.0.4 Added the '$n' form for the replacement parameter
4.0.2 Added the limit parameter

Examples

Example 1726. Using backreferences followed by numeric literals

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<?php
$string = 'April 15, 2003';
$pattern = '/(\w+) (\d+), (\d+)/i';
$replacement = '${1}1,$3';
echo preg_replace($pattern, $replacement, $string);
?>

The above example will output:

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April1,2003


Example 1727. Using indexed arrays with preg_replace()

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<?php
$string = 'The quick brown fox jumped over the lazy dog.';
$patterns[0] = '/quick/';
$patterns[1] = '/brown/';
$patterns[2] = '/fox/';
$replacements[2] = 'bear';
$replacements[1] = 'black';
$replacements[0] = 'slow';
echo preg_replace($patterns, $replacements, $string);
?>

The above example will output:

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The bear black slow jumped over the lazy dog.

By ksorting patterns and replacements, we should get what we wanted.

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<?php
ksort($patterns);
ksort($replacements);
echo preg_replace($patterns, $replacements, $string);
?>

The above example will output:

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The slow black bear jumped over the lazy dog.


Example 1728. Replacing several values

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<?php
$patterns = array ('/(19|20)(\d{2})-(\d{1,2})-(\d{1,2})/',
                  '/^\s*{(\w+)}\s*=/');
$replace = array ('\3/\4/\1\2', '$\1 =');
echo preg_replace($patterns, $replace, '{startDate} = 1999-5-27');
?>

The above example will output:

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$startDate = 5/27/1999


Example 1729. Using the 'e' modifier

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<?php
preg_replace("/(<\/?)(\w+)([^>]*>)/e",
            "'\\1'.strtoupper('\\2').'\\3'",
            $html_body);
?>

This would capitalize all HTML tags in the input text.


Example 1730. Strip whitespace

This example strips excess whitespace from a string.

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<?php
$str = 'foo   o';
$str = preg_replace('/\s\s+/', ' ', $str);
// This will be 'foo o' now
echo $str;
?>


Example 1731. Using the count parameter

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<?php
$count = 0;

echo preg_replace(array('/\d/', '/\s/'), '*', 'xp 4 to', -1 , $count);
echo $count; //3
?>

The above example will output:

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xp***to
3


Notes

Note:

When using arrays with pattern and replacement, the keys are processed in the order they appear in the array. This is not necessarily the same as the numerical index order. If you use indexes to identify which pattern should be replaced by which replacement, you should perform a ksort() on each array prior to calling preg_replace().

See Also
preg_match()
preg_replace_callback()
preg_split()

Related Examples ( Source code ) » preg_replace

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Code Examples / Notes » preg_replace

as
What about strtr:
<?php
$DNA = "AGTCTGCCCTAG";
echo "$DNA\n";
$DNA = strtr($DNA,"AGCT","TCGA");
echo "$DNA\n";
?>
Andreas

alexey lebedev
Wasted several hours because of this:
$str='It&#039;s a string with HTML entities';
preg_replace('~&#(\d+);~e', 'code2utf($1)', $str);
This code must convert numeric html entities to utf8. And it does with a little exception. It treats wrong codes starting with &#0
The reason is that code2utf will be called with leading zero, exactly what the pattern matches - code2utf(039).
And it does matter! PHP treats 039 as octal number.
Try print(011);
Solution:
preg_replace('~&#0*(\d+);~e', 'code2utf($1)', $str);

anonymous
To djurredenboer:
It's because PHP stores array elements in order of their creation.
Use sort() or ksort() functions to change the order of your arrays!

istvan dot csiszar
This is an addition to the previously sent removeEvilTags function. If you don't want to remove the style tag entirely, just certain style attributes within that, then you might find this piece of code useful:
<?php
function removeEvilStyles($tagSource)
{
  // this will leave everything else, but:
$evilStyles = array('font', 'font-family', 'font-face', 'font-size', 'font-size-adjust', 'font-stretch', 'font-variant');
$find = array();
$replace = array();

foreach ($evilStyles as $v)
{
$find[]    = "/$v:.*?;/";
$replace[] = '';
}

return preg_replace($find, $replace, $tagSource);
}
function removeEvilTags($source)
{
$allowedTags = '<h1><h2><h3><h4><h5><a><img><label>'.
       '


<span><sup><sub><ul><li><ol>'.
       '<table><tr><td><th><tbody><div><hr><em><b><i>';
$source = strip_tags(stripslashes($source), $allowedTags);
return trim(preg_replace('/<(.*?)>/ie', "'<'.removeEvilStyles('\\1').'>'", $source));
}
?>

tim
This function has a little quirk.
When you are trying to use backreferences in the pattern, you MUST use \\n, and not $n. $n doesn't work.

flar
Special care needs to be taken, since all regular expressions are "greedy" by default. What does it mean? Example:
Text: 'the big scary house here and the big weird house over there'
Expression '/big(.*)house/'
Matches: 'big scary house here and the big weird house'
In other words, the expression matches everything between the first occurrence of the word "big" and the last occurrence of the word "house".
If you need to preserve the words 'here and the', you need a non-greedy expression.
If you need a non-greedy regular expression in PHP, add the character "U" after the final slash. So our updated example will look like this:
Expression: '/big(.*)house/U'
Match 1: 'big scary house'
Match 2: 'big weird house'
Note that with the non-greedy expression we now got two matches and the words between the matches are preserved (not replaced).

ae
Something innovative for a change ;-) For a news system, I have a special format for links:
"Go to the [Blender3D Homepage|http://www.blender3d.org] for more Details"
To get this into a link, use:
$new = preg_replace('/\[(.*?)\|(.*?)\]/', '<a href="$2" target="_blank">$1</a>', $new);

sg_01
Re: wcc at techmonkeys dot org
You could put this in 1 replace for faster execution as well:
<?php
/*
* Removes all blank lines from a string.
*/
function removeEmptyLines($string)
{
  return preg_replace("/(^[\r\n]*|[\r\n]+)[\s\t]*[\r\n]+/", "\n", $string);
}
?>

steven -a-t- acko dot net
People using the /e modifier with preg_replace should be aware of the following weird behaviour. It is not a bug per se, but can cause bugs if you don't know it's there.
The example in the docs for /e suffers from this mistake in fact.
With /e, the replacement string is a PHP expression. So when you use a backreference in the replacement expression, you need to put the backreference inside quotes, or otherwise it would be interpreted as PHP code. Like the example from the manual for preg_replace:
preg_replace("/(<\/?)(\w+)([^>]*>)/e",
            "'\\1'.strtoupper('\\2').'\\3'",
            $html_body);
To make this easier, the data in a backreference with /e is run through addslashes() before being inserted in your replacement expression. So if you have the string
He said: "You're here"
It would become:
He said: \"You\'re here\"
...and be inserted into the expression.
However, if you put this inside a set of single quotes, PHP will not strip away all the slashes correctly! Try this:
print ' He said: \"You\'re here\" ';
Output: He said: \"You're here\"
This is because the sequence \" inside single quotes is not recognized as anything special, and it is output literally.
Using double-quotes to surround the string/backreference will not help either, because inside double-quotes, the sequence \' is not recognized and also output literally. And in fact, if you have any dollar signs in your data, they would be interpreted as PHP variables. So double-quotes are not an option.
The 'solution' is to manually fix it in your expression. It is easiest to use a separate processing function, and do the replacing there (i.e. use "my_processing_function('\\1')" or something similar as replacement expression, and do the fixing in that function).
If you surrounded your backreference by single-quotes, the double-quotes are corrupt:
$text = str_replace('\"', '"', $text);
People using preg_replace with /e should at least be aware of this.
I'm not sure how it would be best fixed in preg_replace. Because double-quotes are a really bad idea anyway (due to the variable expansion), I would suggest that preg_replace's auto-escaping is modified to suit the placement of backreferences inside single-quotes (which seemed to be the intention from the start, but was incorrectly applied).

santosh patnaik
Once a match is identified, the regular expression engine appears to set aside the matching segment of the target string. A second segment that you expect to match may therefore end up not getting matched:
// Expect 'pa pa pa pa' but get 'pa ma pa ma'
echo preg_replace('`(^|\s)ma(\s|$)`', '$1pa$2', 'ma ma ma ma');
Here the issue can be solved by using a 'lookahead':
// Expect and get 'pa pa pa pa'
echo preg_replace('`(^|\s)ma(?=\s|$)`', '$1pa', 'ma ma ma ma');

dani dot church
Note that it is in most cases much more efficient to use preg_replace_callback(), with a named function or an anonymous function created with create_function(), instead of the /e modifier.  When preg_replace() is called with the /e modifier, the interpreter must parse the replacement string into PHP code once for every replacement made, while preg_replace_callback() uses a function that only needs to be parsed once.

alexandre
Match and replace for arrays. Useful for parsing entire $_POST
<?php
function array_preg_match(array $patterns, array $subjects, &$errors = array()) {
   $errors = array();
   foreach ($patterns as $k => $v) preg_match($v, $subjects[$k]) or $errors[$k] = TRUE;    
   return count($errors) == 0 ? TRUE : FALSE;
}
function array_preg_replace(array $patterns, array $replacements, array $subject) {
   $r = array();
   foreach ($patterns as $k => $v) $r[$k] = preg_replace($v, $replacements[$k], $subject[$k]);    
   return $r+$subject;
}
?>

jhm
It took me a while to figure this one out, but here is a nice way to use preg_replace to convert a hex encoded string back to clear text
<?php
   $text = "PHP rocks!";
   $encoded = preg_replace(
          "'(.)'e"
         ,"dechex(ord('\\1'))"
         ,$text
   );
   print "ENCODED: $encoded\n";
?>
ENCODED: 50485020726f636b7321
<?php
   print "DECODED: ".preg_replace(
      "'([\S,\d]{2})'e"
     ,"chr(hexdec('\\1'))"
     ,$encoded)."\n";
?>
DECODED: PHP rocks!

gabe
It is useful to note that the 'limit' parameter, when used with 'pattern' and 'replace' which are arrays, applies to each individual pattern in the patterns array, and not the entire array.
<?php
$pattern = array('/one/', '/two/');
$replace = array('uno', 'dos');
$subject = "test one, one two, one two three";
echo preg_replace($pattern, $replace, $subject, 1);
?>
If limit were applied to the whole array (which it isn't), it would return:
test uno, one two, one two three
However, in reality this will actually return:
test uno, one dos, one two three

robert
If you're wanting to strip a variety of timestamps, try this:
<?php
$var = "[09:21:32] Testing";
$var = preg_replace('/\W*\d+:\d+(?::\d+)?[a-zA-Z]*[\W\s]*/', '', $var);
echo $var;
?>
Hope this helps :)

pcabc
if (!function_exists('htmlspecialchars_decode')) {
function htmlspecialchars_decode($str)
{
$str = preg_replace('/\\\"/', '"', $str);
return strtr($str, array_flip(get_html_translation_table(HTML_SPECIALCHARS)));
}
}
a little correcting, change back the \" string to "...

iasmin
I thought that someone could use this hyperlink function.
preg_replace is about 6 times faster than ereg_replace. I took the original example from the ereg_replace function page and modified so that it works perfect. I gave a comment of what it matches.
One thing is that I added a space at the beginning so that only links that don't have <a href="" around them or anything else touching will be replaced.
<i>NOTE! I had to break the long lines otherwise I couldn't have posted this. So take the new line out and it will work</i>
<?php
function hyperlink(&$text)
   {
      // match protocol://address/path/file.extension?some=variable&another=asf%
      $text = preg_replace("/\s(([a-zA-Z]+:\/\/)([a-z][a-z0-9_\..-]*
[a-z]{2,6})([a-zA-Z0-9\/*-?&%]*))\s/i", " <a href=\"$1\">$3</a> ", $text);
   
      // match www.something.domain/path/file.extension?some=variable&another=asf%
      $text = preg_replace("/\s(www\.([a-z][a-z0-9_\..-]*
[a-z]{2,6})([a-zA-Z0-9\/*-?&%]*))\s/i", " <a href=\"http://$1\">$2</a> ", $text);
     
      return $text;
   }
?>
Play around with it and see how it works.
Courtesy of AmazingDiscoveries.org
God bless, Iasmin Balaj

lehongviet
I got problem echoing text that contains double-quotes into a text field. As it confuses value option. I use this function below to match and replace each pair of them by smart quotes. The last one will be replaced by a hyphen(-).
It works for me.
function smart_quotes($text) {
$pattern = '/"((.)*?)"/i';
$text = preg_replace($pattern,"&#147;\\1&#148;",stripslashes($text));
$text = str_replace("\"","-",$text);
$text = addslashes($text);
return $text;
}

djurredenboer
I found something strange
<?php
$string = 'The quick brown fox jumped over the lazy dog.';
$patterns[0] = '/quick/';
$patterns[1] = '/brown/';
$patterns[2] = '/fox/';
$replacements[2] = 'bear';
$replacements[1] = 'black';
$replacements[0] = 'slow';
echo preg_replace($patterns, $replacements, $string);
?>
Output The bear black slow jumped over the lazy dog.
But when you swap the replacements like
$replacements[0] = 'slow';
$replacements[2] = 'bear';
$replacements[1] = 'black';
you get The slow bear black jumped over the lazy dog.

mrozenoer
I could not find a function to unescape javascript unicode escapes anywhere (e.g., "\u003c"=>"<").
<?php
function js_uni_decode($s) {
return preg_replace('/\\\u([0-9a-f]{4})/ie', "chr(hexdec('\\1'))",  $s);
}
echo js_uni_decode("\u003c");
?>

mike dot hayward
Hi.
Not sure if this will be a great help to anyone out there, but thought i'd post just in case.
I was having an Issue with a project that relied on $_SERVER['REQUEST_URI']. Obviously this wasn't working on IIS.
(i am using mod_rewrite in apache to call up pages from a database and IIS doesn't set REQUEST_URI). So i knocked up this simple little preg_replace to use the query string set by IIS when redirecting to a PHP error page.
<?
//My little IIS hack :)
if(!isset($_SERVER['REQUEST_URI'])){
$_SERVER['REQUEST_URI'] = preg_replace( '/404;([a-zA-Z]+:\/\/)(.*?)\//i', "/" , $_SERVER['QUERY_STRING'] );
}
?>
Hope this helps someone else out there trying to do the same thing :)
If anyone finds a better way, please let met know, I'm still learning ;)

mac
Here's a derivate of Iasmin's function.
This one handles URL's followed by a period or comma too and adds a _blank target to the hyperlinks.
Additionally it replaces e-mail addresses with a mailto hyperlink.
<?php
function hyperlink($text) {
// match protocol://address/path/file.extension?some=variable&another=asf%
$text = preg_replace("/\s([a-zA-Z]+:\/\/[a-z][a-z0-9\_\.\-]*
[a-z]{2,6}[a-zA-Z0-9\/\*\-\?\&\%]*)([\s|\.|\,])/i",
" <a href=\"$1\" target=\"_blank\">$1</a>$2", $text);
// match www.something.domain/path/file.extension?some=variable&another=asf%
$text = preg_replace("/\s(www\.[a-z][a-z0-9\_\.\-]*
[a-z]{2,6}[a-zA-Z0-9\/\*\-\?\&\%]*)([\s|\.|\,])/i",
" <a href=\"http://$1\" target=\"_blank\">$1</a>$2", $text);
// match name@address
$text = preg_replace("/\s([a-zA-Z][a-zA-Z0-9\_\.\-]*[a-zA-Z]*
\@[a-zA-Z][a-zA-Z0-9\_\.\-]*[a-zA-Z]{2,6})([\s|\.|\,])/i",
" <a href=\"mailto://$1\">$1</a>$2", $text);
return $text;
}
?>

eric
Here recently I needed a way to replace links (<a href="blah.com/blah.php">Blah</a>) with their anchor text, in this case Blah. It might seem simple enough for some..or most, but at the benefit of helping others:
<?php
$value = '<a href="http://www.domain.com/123.html">123</a>';
echo preg_replace('/<a href="(.*?)">(.*?)<\\/a>/i', '$2', $value);
//Output
// 123
?>

kyle
Here is a regular expression to "slashdotify" html links.  This has worked well for me, but if anyone spots errors, feel free to make corrections.
<?php
$url = '<a attr="garbage" href="http://us3.php.net/preg_replace">preg_replace - php.net</a>';
$url = preg_replace( '/<.*href="?(.*:\/\/)?([^ \/]*)([^ >"]*)"?[^>]*>(.*)(<\/a>)/', '<a href="$1$2$3">$4</a> [$2]', $url );
?>
Will output:
<a href="http://us3.php.net/preg_replace">preg_replace - php.net</a> [us3.php.net]

sternkinder
From what I can see, the problem is, that if you go straight and substitute all 'A's wit 'T's you can't tell for sure which 'T's to substitute with 'A's afterwards. This can be for instance solved by simply replacing all 'A's by another character (for instance '_' or whatever you like), then replacing all 'T's by 'A's, and then replacing all '_'s (or whatever character you chose) by 'A's:
$dna = "AGTCTGCCCTAG";
echo str_replace(array("A","G","C","T","_","-"), array("_","-","G","A","T","C"), $dna); //output will be TCAGACGGGATC
Although I don't know how transliteration in perl works (though I remember that is kind of similar to the UNIX command "tr") I would suggest following function for "switching" single chars:
function switch_chars($subject,$switch_table,$unused_char="_") {
   foreach ( $switch_table as $_1 => $_2 ) {
       $subject = str_replace($_1,$unused_char,$subject);
       $subject = str_replace($_2,$_1,$subject);
       $subject = str_replace($unused_char,$_2,$subject);
   }
   return $subject;
}
echo switch_chars("AGTCTGCCCTAG", array("A"=>"T","G"=>"C")); //output will be TCAGACGGGATC

robvdl
For those of you that have ever had the problem where clients paste text from msword into a CMS, where word has placed all those fancy quotes throughout the text, breaking the XHTML validator... I have created a nice regular expression, that replaces ALL high UTF-8 characters with HTML entities, such as &#8217;.
Note that most user examples on php.net I have read, only replace selected characters, such as single and double quotes. This replaces all high characters, including greek characters, arabian characters, smilies, whatever.
It took me ages to get it just downto two regular expressions, but it handles all high level characters properly.
$text = preg_replace('/([\xc0-\xdf].)/se', "'&#' . ((ord(substr('$1', 0, 1)) - 192) * 64 + (ord(substr('$1', 1, 1)) - 128)) . ';'", $text);
$text = preg_replace('/([\xe0-\xef]..)/se', "'&#' . ((ord(substr('$1', 0, 1)) - 224) * 4096 + (ord(substr('$1', 1, 1)) - 128) * 64 + (ord(substr('$1', 2, 1)) - 128)) . ';'", $text);

anon
For the benefit of perl coders,
$s =~ s/PATTERN/REPLACEMENT/g;
becomes:
<?
$s = preg_replace('/PATTERN/', 'REPLACEMENT', $s);
?>
Note that you have to assign the result back to $s.  If your preg_replace doesn't seem to be working, you may have merely forgotten to assign the return to $s.

flar
For completeness, with regard to my previous message, it should be noted that the following expressions are functionally identical in PHP:
'/big(.*)house/U'
'/big(.*?)house/'
Last note: The parenthesis can be omitted.

jon
for a url explode I would suggest parse_url($url). Its far simpler than the list of preg_replaces used.

igasparetto
Displaying results of a search engine:
$words=explode(" ", $_POST['query']);
foreach($words as $word){
$patterns[]='/'.$word.'/i';
$replaces[]='<span class="textFound">$0</span>';
}
// run sql
$display_results="";
foreach($res as $row)
$display_results .= "

" . preg_replace($patterns, $replaces, nl2br(htmlentities( $row['field'] ) ) ) . "\n";
echo $display_results;

ismith
Be aware that when using the "/u" modifier, if your input text contains any bad UTF-8 code sequences, then preg_replace will return an empty string, regardless of whether there were any matches.
This is due to the PCRE library returning an error code if the string contains bad UTF-8.

131 dot php
Based on previous comment, i suggest
( this function already exist in php 6 )
function unicode_decode($str){
return preg_replace(
'#\\\u([0-9a-f]{4})#e',
"unicode_value('\\1')",
$str);
}
function unicode_value($code) {
$value=hexdec($code);
if($value<0x0080)
return chr($value);
elseif($value<0x0800)
return chr((($value&0x07c0)>>6)|0xc0)
.chr(($value&0x3f)|0x80);
else
return chr((($value&0xf000)>>12)|0xe0)
.chr((($value&0x0fc0)>>6)|0x80)
.chr(($value&0x3f)|0x80);
}

matt
at below post:
<?php
$template = "Price: #price#";
$price    = '&#36;5';
print "Price: $price\n";
$res    = preg_replace("/#price#/", $price, $template);
print "From template: -> $res\n";
?>

anzenews
As steven -a-t- acko dot net explained, using /e modifier is tricky if strings have quotes in them. However, the solution might be easier than manual coding of some sort of stripslashes - just use preg_replace_callback instead of preg_replace. It is safer anyway.

rob
Also worth noting is that you can use array_keys()/array_values() with preg_replace like:
$subs = array(
 '/\[b\](.+)\[\/b\]/Ui' => '<strong>$1</strong>',
 '/_(.+)_/Ui' => '<em>$1</em>'
 ...
 ...
);
$raw_text = '[b]this is bold[/b] and this is _italic!_';
$bb_text = preg_replace(array_keys($subs), array_values($subs), $raw_text);

santosh patnaik
@giel dot berkers
Use the 'PCRE_DOTALL' ('s') option so that the '.' covers newline characters:
$code = preg_replace('/\/\*.*\*\//ms', '', $code);

jefkin
@ Santosh Patnaik
The perl regular expression engine will handle this expression better and much faster by using the word boundry escape code \b.  Though it may not be obvious except to long time perl geeks such as I :)
so:
// Expect and get 'pa pa pa pa'
echo preg_replace('`\bma\b`', 'pa', 'ma ma ma ma');
Jeff

info
@ Mac: Your adjustment only seems to work when the period or comma is followed by a newline. The code breaks when more characters follow.
Try the following code instead. It parses the url to a point where all punctuation is followed by either a word character or a slash.
<?php
$string = preg_replace('#(^|\s)([a-z]+://([^\s\w/]?[\w/])*)#is', '\\1<a href="\\2">\\2</a>', $string);
$string = preg_replace('#(^|\s)((www|ftp)\.([^\s\w/]?[\w/])*)#is', '\\1<a href="http://\\2">\\2</a>', $string);
$string = preg_replace('#(^|\s)(([a-z0-9._%+-]+)@(([.-]?[a-z0-9])*))#is', '\\1<a href="mailto:\\2">\\2</a>', $string);
?>
This seems fairly waterproof but please post your improvements.

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