Unexpected behavior around recursive regex

Question

I am trying to match C++ argument type which can contain balanced <and > characters.

With this regex: (\<(?>[^<>]|(?R))*\>)

On this string: QMap<QgsFeatureId, QPair<QMap<Something, Complex> >>

It matches all expect the first 4 characters (QMap).

Now, if I add \w+ at the start of my regex, it now only matches the end of it (QPair<QMap<Something, Complex> >>) and not the whole string.

What is the explanation and how to solve this?

You can try it online here.

This is intented to use in Perl 5.10+ (5.24).

What is the expected result you need to get in the end? `QMap` as Group 1 and ` >>` as Group 2? — Wiktor Stribiżew, Mar 30 '17 at 06:34

score 4 · Accepted Answer · answered Mar 30 '17 at 06:36

The (?R) construct recurses the entire pattern. When you add \w+ at the start, it is also accounted for when the recursion takes place. However, what you want to recurse is the Group 1 subpattern.

You need a subroutine call that will recurse the capturing group subpattern:

(\w+)(<(?:[^<>]++|(?2))*>)

See the regex demo

Details

(\w+) - Group 1 capturing the identifier (you may change it to [a-zA-Z]\w*)
(<(?:[^<>]++|(?2))*>) - Group 2 (that will be recursed)
- < - a literal <
- (?:[^<>]++|(?2))* - either 1+ chars other than < and > (possessively, to make it faster) or (|) the whole Group 2 pattern ((?2)).
- > - a literal >

Results:

Match:   QMap<QgsFeatureId, QPair<QMfap<Something, Complex> >>
Group 1: QMap
Group 2: <QgsFeatureId, QPair<QMfap<Something, Complex> >>

awesome, thanks for the details. Btw, would be nice to add tagg regex-recursive, which I can't do due to my low rank. — Denis Rouzaud, Mar 30 '17 at 06:50
Welll, maybe it is a good idea. Something like `regex-recursion` or `regex-recursive`. — Wiktor Stribiżew, Mar 30 '17 at 06:53

Unexpected behavior around recursive regex

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