[ruby-core:122722] [Ruby Bug#21507] Regexp considers variable repetition quantifiers invalid in lookbehind

11 Jul 2025

      Issue #21507 has been reported by tiago-macedo (Tiago Macedo).

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Bug #21507: Regexp considers variable repetition quantifiers invalid in lookbehind
https://bugs.ruby-lang.org/issues/21507

* Author: tiago-macedo (Tiago Macedo)
* Status: Open
* ruby -v: ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-linux-gnu]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
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This is my first bug subscription, please feel free to tell me if I can do anything better.

# Description

Attempting to use "variable" repetition quantifiers (`?`, `+`,`*`,`{n,}`, ...) inside lookbehind anchors raises a **SyntaxError** (invalid pattern in look-behind), but it's perfectly viable to do it in lookafter anchors.

Examples of lookafter working:
```ruby
irb(main):100> "axb".split /(?=x)/
=> ["a", "xb"]
irb(main):101> "axb".split /(?=x?)/
=> ["a", "x", "b"]
irb(main):102> "axb".split /(?=x+)/
=> ["a", "xb"]
irb(main):103> "axb".split /(?=x*)/
=> ["a", "x", "b"]
irb(main):104> "axb".split /(?=x{1,})/
=> ["a", "xb"]
irb(main):105> "axb".split /(?=x{,1})/
=> ["a", "x", "b"]
irb(main):106> "axb".split /(?=x{1,2})/
=> ["a", "xb"]
```

Examples of lookbehind **working only with non-variable metacharacters**:
```ruby
irb(main):107> "axb".split /(?<=x)/
=> ["ax", "b"]
irb(main):108> "axb".split /(?<=x?)/
/var/lib/gems/3.0.0/gems/irb-1.14.0/exe/irb:9:in `<top (required)>': (irb):108: invalid pattern in look-behind: /(?<=x?)/ (SyntaxError)
	from /usr/local/bin/irb:25:in `load'
	from /usr/local/bin/irb:25:in `<main>'
irb(main):109> "axb".split /(?<=x*)/
/var/lib/gems/3.0.0/gems/irb-1.14.0/exe/irb:9:in `<top (required)>': (irb):109: invalid pattern in look-behind: /(?<=x*)/ (SyntaxError)
	from /usr/local/bin/irb:25:in `load'
	from /usr/local/bin/irb:25:in `<main>'
irb(main):110> "axb".split /(?<=x{1,})/
/var/lib/gems/3.0.0/gems/irb-1.14.0/exe/irb:9:in `<top (required)>': (irb):110: invalid pattern in look-behind: /(?<=x{1,})/ (SyntaxError)
	from /usr/local/bin/irb:25:in `load'
	from /usr/local/bin/irb:25:in `<main>'
irb(main):111> "axb".split /(?<=x{,1})/
/var/lib/gems/3.0.0/gems/irb-1.14.0/exe/irb:9:in `<top (required)>': (irb):111: invalid pattern in look-behind: /(?<=x{,1})/ (SyntaxError)
	from /usr/local/bin/irb:25:in `load'
	from /usr/local/bin/irb:25:in `<main>'
irb(main):112> "axb".split /(?<=x{1,2})/
/var/lib/gems/3.0.0/gems/irb-1.14.0/exe/irb:9:in `<top (required)>': (irb):112: invalid pattern in look-behind: /(?<=x{1,2})/ (SyntaxError)
	from /usr/local/bin/irb:25:in `load'
	from /usr/local/bin/irb:25:in `<main>'
irb(main):113> "axb".split /(?<=x{1})/
=> ["ax", "b"]
irb(main):114> "axb".split /(?<=x{1,1})/
=> ["ax", "b"]
```

# Note

I have searched on the internet and, to my knowledge, this behavior is not intended. (This documentation page on regular expressions)[https://ruby-doc.org/core-3.1.0/doc/regexp_rdoc.html], for example, does not say anything about limitations specific to lookbehinds.

-- 
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