recursive regular expression to process nested strings enclosed by {| and |}

Question

In a project I have a text with patterns like that:

{| text {| text |} text |}
more text

I want to get the first part with brackets. For this I use preg_match recursively. The following code works fine already:

preg_match('/\{((?>[^\{\}]+)|(?R))*\}/x',$text,$matches);

But if I add the symbol "|", I got an empty result and I don't know why:

preg_match('/\{\|((?>[^\{\}]+)|(?R))*\|\}/x',$text,$matches);

I can't use the first solution because in the text something like { text } can also exist. Can somebody tell me what I do wrong here? Thx

You can use Balancing groups in .NET like described here: marcomilani.it/2012/07/… — Marco
– Marco, Commented Jul 17, 2012 at 11:46

Alan Moore · Accepted Answer · 2009-12-14 08:09:12Z

3

Try this:

'/(?s)\{\|(?:(?:(?!\{\||\|\}).)++|(?R))*\|\}/'

In your original regex you use the character class [^{}] to match anything except a delimiter. That's fine when the delimiters are only one character, but yours are two characters. To not-match a multi-character sequence you need something this:

(?:(?!\{\||\|\}).)++

The dot matches any character (including newlines, thank to the (?s)), but only after the lookahead has determined that it's not part of a {| or |} sequence. I also dropped your atomic group ((?>...)) and replaced it with a possessive quantifier (++) to reduce clutter. But you should definitely use one or the other in that part of the regex to prevent catastrophic backtracking.

answered Dec 14, 2009 at 8:09

Alan Moore

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Prog Over a year ago

I just tried your solution and it works well. Thank you very much! And also thanks for the explanation, because it's not easy to understand.

outis · Accepted Answer · 2009-12-14 13:23:02Z

1

You've got a few suggestions for working regular expressions, but if you're wondering why your original regexp failed, read on. The problem lies when it comes time to match a closing "|}" tag. The (?>[^{}]+) (or [^{}]++) sub expression will match the "|", causing the |} sub expression to fail. With no backtracking in the sub expression, there's no way to recover from the failed match.

answered Dec 14, 2009 at 13:23

outis

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Community · Accepted Answer · 2017-05-23 10:32:51Z

0

See PHP - help with my REGEX-based recursive function

To adapt it to your use

preg_match_all('/\{\|(?:^(\{\||\|\})|(?R))*\|\}/', $text, $matches);

edited May 23, 2017 at 10:32

CommunityBot

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answered Dec 14, 2009 at 4:23

Tor Valamo

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