PHP Preg_Replace Example

The following is a simple example of the PHP function preg_replace(). It's also a short tutorial on regex (regular expressions) using Perl since this is the regex patterns the preg_replace function utilizes.

Description

mixed preg_replace ( mixed pattern, mixed replacement, mixed subject [, int limit [, int &count]] )

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

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.

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.

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().

Parameters

pattern

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

replacement

The string or an array with strings to replace. If this parameter is a string and the pattern parameter is an array, all pattens 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 less keys in the replacement array than in the pattern array, the excedent patterns will be replaced by an empty string.

subject

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

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
4.0.1pl2	Added the limit parameter
4.0.4	Added the '$n' form for the replacement parameter
5.1.0	Added the count parameter

Examples

Example 1. Convert HTML to text <?php // $document should contain an HTML document. // This will remove HTML tags, javascript sections // and white space. It will also convert some // common HTML entities to their text equivalent. $search = array ('@<script[^>]*?>.*?</script>@si', // Strip out javascript                  '@<[\/\!]*?[^<>]*?>@si',          // Strip out HTML tags                  '@([\r\n])[\s]+@',                // Strip out white space                  '@&(quot|#34);@i',                // Replace HTML entities                  '@&(amp|#38);@i',                  '@&(lt|#60);@i',                  '@&(gt|#62);@i',                  '@&(nbsp|#160);@i',                  '@&(iexcl|#161);@i',                  '@&(cent|#162);@i',                  '@&(pound|#163);@i',                  '@&(copy|#169);@i',                  '@&#(\d+);@e');                    // evaluate as php $replace = array ('',                  '',                  '\1',                  '"',                  '&',                  '<',                  '>',                  ' ',                  chr(161),                  chr(162),                  chr(163),                  chr(169),                  'chr(\1)'); $text = preg_replace($search, $replace, $document); ?>

Example 2. Using backreferences followed by numeric literals <?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: April1,2003

Example 3. Using indexed arrays with preg_replace() <?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, $string); ?> The above example will output: The bear black slow jumped over the lazy dog. By ksorting patterns and replacements, we should get what we wanted. <?php ksort($patterns); ksort($replacements); echo preg_replace($patterns, $replacements, $string); ?> The above example will output: The slow black bear jumped over the lazy dog.

Example 4. Replacing several values <?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: $startDate = 5/27/1999

Example 5. Using the 'e' modifier <?php preg_replace("/(<\/?)(\w+)([^>]*>)/e",              "'\\1'.strtoupper('\\2').'\\3'",              $html_body); ?> This would capitalize all HTML tags in the input text.

Example 6. Strip whitespace This example strips excess whitespace from a string. <?php $str = 'foo  o'; $str = preg_replace('/\s\s+/', ' ', $str); // This will be 'foo o' now echo $str; ?>

Example 7. Using the count parameter <?php $count = 0; echo preg_replace(array('/\d/', '/\s/'), '*', 'xp 4 to', -1 , $count); echo $count; //3 ?> The above example will output: xp***to 3