Recursive nested matching pairs of curly braces in Ruby regex

Question

I have the following string:

The {quick} brown fox {jumps {over {deep} the} {sfsdf0} lazy} dog {sdfsdf1 {sdfsdf2}

And PHP Regular Expression:

/(?=\{((?:[^{}]+|\{(?1)\})+)\})/g

It yields the following matches:

[5-10]  `quick`
[23-60] `jumps {over {deep} the} {sfsdf} lazy`
[30-45] `over {deep} the`
[36-40] `deep`
[48-54] `sfsdf0`
[76-83] `sdfsdf2`

See: http://regex101.com/r/fD3iZ2.

I'm trying to get the equivalent working in Ruby, but I'm having a problem with (?1)… resulting in an undefined group option error:

str = "The {quick} brown fox {jumps {over {deep} the} {sfsdf} lazy} dog {sdfsdf {sdfsdf}"
str.scan /(?=\{((?:[^{}]+|\{(?1)\})+)\})/

SyntaxError: undefined group option: /(?=\{((?:[^{}]+|\{(?1)\})+)\})/

See: http://fiddle.re/n6w4n.

Coincidently, I get the same sort of error in Javascript and Python.

My regex foo is nearly exhausted today, any help very much appreciated.

Answer 1

Ruby uses a different syntax for recursion: \g<1> replaces (?1). So try

(?=\{((?:[^{}]++|\{\g<1>\})++)\})

I also made the quantifiers possessive to avoid excessive backtracking in case of unbalanced braces.

irb(main):003:0> result = str.scan(/(?=\{((?:[^{}]++|\{\g<1>\})++)\})/)
=> [["quick"], ["jumps {over {deep} the} {sfsdf} lazy"], ["over {deep} the"], 
["deep"], ["sfsdf"], ["sdfsdf"]]