Regular expression backreference non-capturing group?

Question

Against a string like this:

<h3>title</h3>
<h4>title</h4>

How to match the tags correspondingly and get the text in them?

This works but it unnecessarily gets the tag name:

'@<(h[34])>(.+)</\1>@sU'

However this doesn't seem to work as I don't want to get the tag name but just want to backreference it:

'@<(?:h[34])>(.+)</\1>@sU'

I'm using PHP preg_match(). Why doesn't the 2nd approach work? Is it possible to back reference a non-capturing group?

Answer 1

Capturing groups could be used later on in the regular expression as a backreference to what was matched in that captured group. By placing ?: inside you specify that the group is not to be captured, but to group expressions.

You can use the branch reset feature (?| ... | ... ) that way you don't have your expression matching non-corresponding tags and both capturing groups in the alternatives are considered as one capturing group.

~(?|<h3>(.+?)</h3>|<h4>(.+?)</h4>)~s

Live Demo