Recursive Regex with a Pattern Matching only on Start of Match before Recursion?

Question

I'm trying to find matching parentheses where there are also some more in the middle. I have the following regex that does that, it matches the parenthesis to find the one associated with it. What I need now is that it also searches a prefix for example "Test". It however recurses the entire pattern. I can't simply include "Test" in the start of the pattern. I also only want it to match when the prefix starts with "Test" and not just match any parentheses.

I want to replace the "Test( ... )" with something else, so it'd be preferable if it was one pattern so I can just use regex.sub().

import regex

# want to match 'Test(' prefix specifically
regex.search('\(([^()]|(?R))*\)', '... () ... Test(123, Test(123, (3), 3))')

Wiktor Stribiżew · Accepted Answer · 2018-05-22 21:03:13Z

1

That is the case when you need to use subroutines. Here, you need to enclose the recursed pattern in a capturing group and then use (?1) construct to recurse it:

import regex
m = regex.search(r'Test(\((?:[^()]++|(?1))*\))', 'Test(123, Test(123, (3), 3))')
if m:
    print(m.group()) # => Test(123, Test(123, (3), 3))

See the Python demo.

Details

Test - a prefix word
(\((?:[^()]++|(?1))*\)) - Capturing group 1 (that will be recursed with (?1)):
- \( - a ( char
- (?:[^()]++|(?1))* - zero or more reptitions of
  - [^()]++ - 1+ chars other than ( and ) (possessive match for better efficiency)
  - | - or
  - (?1) - a subroutine to recurse Capturing group #1 subpattern
- \) - a ) char.

answered May 22, 2018 at 21:03

Wiktor Stribiżew

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