From: shortcutter@...
Date: 2016-10-01T09:04:01+00:00
Subject: [ruby-core:77464] [Ruby trunk Bug#12728] Negative lookahead does not work for "+" even though works for "@"

Issue #12728 has been updated by Robert Klemme.

File rx-mini.rb added

Yui NARUSE wrote:
> In case 2, the regexp just behave as if
> 
> ```
> t %r{
>     (?<!\\)\(                           # outer bracket
>     o\+
> 
>     (?<!\\) ([+*]|\{\d+,\}) (?!\+)  # inner repetition, non possessive
> 
>     .*
>     (?<!\\)\)                           # outer bracket
>     (?<!\\) (?:[+*]|\{\d+,\}) # unbounded repetition, non possessive
> 
>   }x, "f(o++)+"
> ```
> 
> Of course it matches.

Argh!  Stupid me.  Yes, the negative lookahead will also match with the closing bracket.

> Maybe use should `[a-zA-Z0-9]*` or something instead of `.*`.

I included a closing bracket in the negative lookahead, then it works:

$ diff -U3 x1 x2
--- x1	2016-10-01 10:55:50.595060831 +0200
+++ x2	2016-10-01 10:55:44.459048792 +0200
@@ -13,20 +13,19 @@
 
   /x
 
-MATCH
+NO MATCH
 s = 'f(o++)+'
 rx = /
     (?<!\\)\(                           # outer bracket
     (.*)
 
-    (?<!\\) ([+*]|\{\d+,\}) (?!\+)  # inner repetition, non possessive
+    (?<!\\) ([+*]|\{\d+,\}) (?!\+|\))  # inner repetition, non possessive
 
     (.*)
     (?<!\\)\)                           # outer bracket
     (?<!\\) (?:[+*]|\{\d+,\}) # unbounded repetition, non possessive
 
   /x
-match = #<MatchData "(o++)+" 1:"o+" 2:"+" 3:"">
 
 MATCH
 s = 'f(o++)+'

The following .* blinded me for the fact that the closing bracket can match with \) AND fulfill the lookahead like in

irb(main):001:0> /a(?=b)b/.match "abc"
=> #<MatchData "ab">

I am sorry for the hassle.

Kind regards

robert

PS: Attaching the test version that produced output x2.

----------------------------------------
Bug #12728: Negative lookahead does not work for "+" even though works for "@"
https://bugs.ruby-lang.org/issues/12728#change-60735

* Author: Robert Klemme
* Status: Rejected
* Priority: Normal
* Assignee: 
* ruby -v: ruby 2.2.5p319 (2016-04-26 revision 54774) [x86_64-cygwin]
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
----------------------------------------
I'll attach a test program that shows the effect.  Basically, if I have a negative lookahead in the regex like (?!@) and "@" shows up in the proper location I get a mismatch (1. case).  This is expected.  If I exchange the "@" with a "\+" or "[+]" in the regex and a "+" in the input, a match occurs (case 2 and 3).  This is the bug.  If the "+" or "@" is removed from the string an expected match occurs (case 4 and 5).  I was not able to boil this down to a smaller example yet.

---Files--------------------------------
rx-mini.rb (1.63 KB)
rx-mini.rb (1.65 KB)


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