16

How can I replace a substring in a found pattern, but leaving the rest as it is?

(EDIT: The real case is of course more complicated than the example below, I have to match occurrences within xml tags. That's why I have to use regex!)

Let's say I want to change occurrences of the letter "X" within a word to the letter "Z".

I want

aaXaa aaX Xaa

to become

aaZaa aaZ Zaa

Finding occurrences of words including "x" isn't a problem, like this:

[^X\s]X[^\s]

but a normal preg_match replaces the complete match, where I want anything in the pattern except "X" to stay as it is.

Which is the best way to accomplish this in php?

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3
  • Your regex would not match the X at the end of the second word. Commented Jan 24, 2010 at 11:37
  • I'm confused, doesn't a simple replace do exactly what you ask? Replacing the matching pattern and leaving the rest as is. Commented Jan 24, 2010 at 11:45
  • Sorry for cunfusing. I have to user regex, my example was a bit over-simplified! Commented Jan 24, 2010 at 11:56

5 Answers 5

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15

If your regex matches only the relevant part, it should be no problem that it replaces the complete match (like preg_replace('/X/', 'Z', $string)).

But if you need the regex to contain parts that should not be replaced, you need to capture them and insert them back:

preg_replace('/(non-replace)X(restofregex)/', '$1Z$2', $string);
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3 Comments

Thank you, Soulmerge! Exactly what I needed!
I found when concatenating a string variable between the two matching groups I had to put the numbers in curly brackets, like this '${1}' . $foo . '${2}', otherwise it didn't work. Why is that?
If instead of Z you need to put value from a variable that you do not control, then you need to escape the $ and \number characters, and this is quite a difficult task. Use preg_replace_callback, an example is in my answer to the question.
3

If it's really as simple as replacing X with Z, you can also use str_replace(), which is faster than using preg in this case:

$sNew = str_replace("X", "Z", $sOld);
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1 Comment

I get your point, and the real case is of course more complicated. I will use it for handling namespace tags in xml-documents, so there has to be real regex for this!
1

Try this

<?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.
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1 Comment

You need to ksort the arrays before you call preg_replace, so that the right patterns get substituted by the right values: ksort($patterns); ksort($replacements);
0

Do you really have to use regex for this?

$output = str_replace('X', 'Z', $input);
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0

For difficult regular expressions where you need to replace part of a string, it can be very useful to do so (and most importantly, you don't need to escape $newValue for the characters $ and \number):

$newValue = 'NewValue';

$newString = preg_replace_callback(
  '/(<Element>).*(<\/Element>)/',
  function($match) use ($newValue) { return $match[1].$newValue.$match[2]; },
  $string
);

Unreliable usage:

In preg_replace('/(<Element>).*(<\/Element>)/', '$1NewValue$2', $string): if instead of NewValue you need to put value from a variable that you do not control, then you need to escape the $ and \number characters, and this is quite a difficult task.

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