Avoid backreference replacement in php's preg_replace

Question

Consider the below use of preg_replace

$str='{{description}}';
$repValue='$0.0 $00.00 $000.000 $1.1 $11.11 $111.111';

$field = 'description';
$pattern = '/{{'.$field.'}}/';

$str =preg_replace($pattern, $repValue, $str );
echo $str;


// Expected output: $0.0 $00.00 $000.000 $1.1 $11.11 $111.11
// Actual output:   {{description}}.0 {{description}}.00 {{description}}0.000 .1 .11 1.111

Here is a phpFiddle showing the issue

It's clear to me that the actual output is not as expected because preg_replace is viewing $0, $0, $0, $1, $11, and $11 as back references for matched groups replacing $0 with the full match and $1 and $11 with an empty string since there are no capture groups 1 or 11.

How can I prevent preg_replace from treating prices in my replacement value as back references and attempting to fill them?

Note that $repValue is dynamic and it's content will not be know before the operation.

Are you sure you need to use preg_replace, not str_replace? — Barmar
– Barmar, Commented Oct 11, 2016 at 0:05
@Barmar , you're right, I could actually just use str_replace, thank you — Wesley Smith
– Wesley Smith, Commented Oct 11, 2016 at 0:42

Casimir et Hippolyte · Accepted Answer · 2016-10-11 01:10:25Z

5

Escape the dollar character before using a character translation (strtr):

$repValue = strtr('$0.0 $00.00 $000.000 $1.1 $11.11 $111.111', ['$'=>'\$']);

For more complicated cases (with dollars and escaped dollars) you can do this kind of substitution (totally waterproof this time):

$str = strtr($str, ['%'=>'%%', '$'=>'$%', '\\'=>'\\%']);
$repValue = strtr($repValue, ['%'=>'%%', '$'=>'$%', '\\'=>'\\%']);
$pattern = '/{{' . strtr($field, ['%'=>'%%', '$'=>'$%', '\\'=>'\\%']) . '}}/';
$str = preg_replace($pattern, $repValue, $str );
echo strtr($str, ['%%'=>'%', '$%'=>'$', '\\%'=>'\\']);

Note: if $field contains only a literal string (not a subpattern), you don't need to use preg_replace. You can use str_replace instead and in this case you don't have to substitute anything.

edited Oct 11, 2016 at 1:10

answered Oct 10, 2016 at 23:43

Casimir et Hippolyte

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3 Comments

Deep Over a year ago

No. Just specific preg_quote()

Casimir et Hippolyte Over a year ago

@Deep: no, preg_quote is only designed for patterns and is not appropriate for replacement strings (make the test)

Deep Over a year ago

Ou... for replacement. Sorry.

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