php/regex: How to replace part of a found pattern, but leaving the rest as it is?

Question

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?

Answer 1

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);