PCRE regex to remove empty braces

Question

How can one write a regex to remove all pairs of braces that contain nothing?

For example, {} and {{}} should be reduced to an empty string, but {{} becomes { and {{A}{}} becomes {{A}}.

I'm currently running s/\{\}//g in a loop until the string length is fixed, but is there a better way to do this?

Matching balanced pairs is one of the most difficult regex problems. Fortunately, PCRE has an extension to deal with it. This answer might help. https://stackoverflow.com/questions/5410652/regex-delete-contents-of-square-brackets — Schwern, Aug 01 '15 at 04:55

score 10 · Accepted Answer · edited Aug 01 '15 at 16:35

10

Matching balanced pairs using traditional regular expressions is difficult, if not impossible. Fortunately PCRE and others have an extension to match recursively, (?R) will recursively match the entire pattern.

/\{(?R)*\}/

That says to match brace pairs which have zero or more brace pairs inside them. See perlretut->Recursive patterns and perlre->Extended Patterns->?R for more information.

edited Aug 01 '15 at 16:35

Håkon Hægland

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answered Aug 01 '15 at 05:06

Schwern

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score 2 · Answer 2 · answered Aug 01 '15 at 13:24

2

Without recursion:

1 while s/\{\}//g;

answered Aug 01 '15 at 13:24

Borodin

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PCRE regex to remove empty braces

2 Answers2