php之preg_replace详解

preg_replace

(PHP 4, PHP 5)

preg_replace — Perform a regular expression search and replace

Description

mixed  preg_replace (  mixed $pattern ,  mixed $replacement ,  mixed $subject [,  int $limit = -1[,  int &$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.

Several PCRE modifiers are also available, including the deprecated 'e' (PREG_REPLACE_EVAL), which is specific to this function.

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. To use backslash in replacement, it must be doubled ("\\\\" PHP string).

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 \\1notation 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 deprecated 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.

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 or NULLif an error occurred.

Errors/Exceptions

An E_DEPRECATED level error is emitted when passing in the "\e" modifier.

Changelog

Version	Description
5.5.0	The /e modifier is deprecated. Use preg_replace_callback() instead. See the PREG_REPLACE_EVALdocumentation for additional information about security risks.
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 #1 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 #2 Using indexed arrays with preg_replace()

<?php
$string = 'The quick brown fox jumped over the lazy dog.';
$patterns = array();
$patterns[0] = '/quick/';
$patterns[1] = '/brown/';
$patterns[2] = '/fox/';
$replacements = array();
$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 #3 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 #4 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 #5 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

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 whichpattern should be replaced by which replacement, you should perform a ksort() on each array prior to calling preg_replace().

See Also

• PCRE Patterns
• preg_filter() - Perform a regular expression search and replace
• preg_match() - Perform a regular expression match
• preg_replace_callback() - Perform a regular expression search and replace using a callback
• preg_split() - Split string by a regular expression
• preg_last_error() - Returns the error code of the last PCRE regex execution