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preg_replace
Perform a regular expression search and replace
Description
mixed preg_replace
( mixed $pattern
, mixed $replacement
, mixed $subject
[, int $limit = -1
[, int &$count
]] )
Parameters
-
pattern
-
The pattern to search for. It can be either a string or an array with
strings.
Several PCRE modifiers
are also available.
-
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
$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
\\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 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 NULL if an error occurred.
Errors/Exceptions
As of PHP 5.5.0 E_DEPRECATED level error is emitted when
passing in the "\e" modifier. As of PHP 7.0.0 E_WARNING is
emitted in this case and "\e" modifier has no effect.
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:
Example #2 Using indexed arrays with preg_replace
<?php
$string = 'The quick brown fox jumps 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, $string);
?>
The above example will output:
The bear black slow jumps over the lazy dog.
By ksorting patterns and replacements, we should get what we wanted.
<?php
ksort($patterns);
ksort($replacements);
echo preg_replace($patterns, $replacements, $string);
?>
The above example will output:
The slow black bear jumps 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:
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:
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
- PCRE Patterns
- preg_quote
- preg_filter
- preg_match
- preg_replace_callback
- preg_split
- preg_last_error
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