From: Ammar Ali <ammarabuali@...>
Date: 2010-11-09T18:42:38+09:00
Subject: Regexp ambiguity/error

Hello,

This is definitely an edge case, but an interesting one. Suppose we
have a configuration string like "mux:off mix:on" and we want to match
the "mix:on" (actually either) part passively (i.e. not captured).


>> "mux:on mix:off" =~ /\b(?mix:(on|off))\b/
=> 4

Oops, forgot about options. 100% understandable.


Several attempts to work around it

Wrap one of the option characters in a set

>> "mux:on mix:off" =~ /\b(?[m]ix:(on|off))\b/
SyntaxError: (irb):2: undefined group option: /\b(?[m]ix:(on|off))\b/
        from /Users/ammar/.rvm/rubies/ruby-1.9.2-p0/bin/irb:17:in `<main>'

Wrap all option characters in individual sets

>> "mux:on mix:off" =~ /\b(?[m][i][x]:(on|off))\b/
SyntaxError: (irb):3: undefined group option: /\b(?[m][i][x]:(on|off))\b/
        from /Users/ammar/.rvm/rubies/ruby-1.9.2-p0/bin/irb:17:in `<main>'

Wrap the option characters in a group

>> "mux:on mix:off" =~ /\b(?(mix):(on|off))\b/
SyntaxError: (irb):4: undefined group option: /\b(?(mix):(on|off))\b/
        from /Users/ammar/.rvm/rubies/ruby-1.9.2-p0/bin/irb:17:in `<main>'

Move the anchors inside the passive group

>> "mux:on mix:off" =~ /(?\b(([m]ix):(on|off))\b)/
SyntaxError: (irb):5: undefined group option: /(?\b(([m]ix):(on|off))\b)/
        from /Users/ammar/.rvm/rubies/ruby-1.9.2-p0/bin/irb:17:in `<main>'


If the passivity is dropped, it works as intended, of course.

>> "mux:on mix:off" =~ /\b(mix:(on|off))\b/
=> 7

The same occurs with ruby 1.8, but with a slightly different error message:

  undefined (?...) sequence:

I was hoping that one of the workarounds above would work, but now I'm
thinking this is might be a bug. I want to see what others think
before I cry wolf.

Regards,
Ammar