[ruby-core:82813] [Ruby trunk Bug#13892] Matching the end of a string followed by an empty greedy regex and a word boundary (.*\b) fails in all versions >= 1.9

Issue #13892 has been updated by jhriggs (Jim Riggs).


@naruse, k-takata -

Testing with Onigmo, Oniguruma, and php's mb_ereg() this case does indeed fail to match, though based on my understanding of this particular pattern (and all of the other regex implementations cited), it should match.

I opened the following upstream issues. I'm not sure if you want to close this or leave it open until fixed upstream.

* https://github.com/k-takata/Onigmo/issues/96
* https://github.com/kkos/oniguruma/issues/70

----------------------------------------
Bug #13892: Matching the end of a string followed by an empty greedy regex and a word boundary (.*\b)  fails in all versions >= 1.9
https://bugs.ruby-lang.org/issues/13892#change-66692

* Author: jhriggs (Jim Riggs)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 1.8.7, 1.9.3p551, 2.0.0p648, 2.1.9p490, 2.2.7p470, 2.3.4p301, 2.4.1p111
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
This is a very specific regex failure that occurs when the final character of the string is matched by the end of a pattern that terminates with `.*\b`. For example:

~~~ ruby
"abc" =~ /c.*\b/
"abc" =~ /abc.*\b/
"abc" =~ /\b.*abc.*\b/
~~~

In Ruby 1.8.7 and every other language I have tested (perl, pcre, javascript, python, go, ...) this matches. Starting in 1.9, it appears that the greedy `.*` causes the `\b` to fail, though it should match. This only occurs when the pattern matches at the end of the string being matched. Based on my non-exhaustive testing, this only occurs with `.*\b`; other patterns like`.?\b` and specific characters such as `d*\b` work as expected:

~~~
ruby1.8 : /c.*\b/       : true
ruby1.8 : /abc.*\b/     : true
ruby1.8 : /\b.*abc.*\b/ : true
ruby1.8 : /c.?\b/       : true
ruby1.8 : /abc.?\b/     : true
ruby1.8 : /\b.?abc.?\b/ : true
ruby1.8 : /d*\b/        : true
ruby1.8 : /abcd*\b/     : true
ruby1.8 : /\b.*abcd*\b/ : true

ruby1.9 : /c.*\b/       : false
ruby1.9 : /abc.*\b/     : false
ruby1.9 : /\b.*abc.*\b/ : false
ruby1.9 : /c.?\b/       : true
ruby1.9 : /abc.?\b/     : true
ruby1.9 : /\b.?abc.?\b/ : true
ruby1.9 : /d*\b/        : true
ruby1.9 : /abcd*\b/     : true
ruby1.9 : /\b.*abcd*\b/ : true

ruby2.0 : /c.*\b/       : false
ruby2.0 : /abc.*\b/     : false
ruby2.0 : /\b.*abc.*\b/ : false
ruby2.0 : /c.?\b/       : true
ruby2.0 : /abc.?\b/     : true
ruby2.0 : /\b.?abc.?\b/ : true
ruby2.0 : /d*\b/        : true
ruby2.0 : /abcd*\b/     : true
ruby2.0 : /\b.*abcd*\b/ : true

ruby2.1 : /c.*\b/       : false
ruby2.1 : /abc.*\b/     : false
ruby2.1 : /\b.*abc.*\b/ : false
ruby2.1 : /c.?\b/       : true
ruby2.1 : /abc.?\b/     : true
ruby2.1 : /\b.?abc.?\b/ : true
ruby2.1 : /d*\b/        : true
ruby2.1 : /abcd*\b/     : true
ruby2.1 : /\b.*abcd*\b/ : true

ruby2.2 : /c.*\b/       : false
ruby2.2 : /abc.*\b/     : false
ruby2.2 : /\b.*abc.*\b/ : false
ruby2.2 : /c.?\b/       : true
ruby2.2 : /abc.?\b/     : true
ruby2.2 : /\b.?abc.?\b/ : true
ruby2.2 : /d*\b/        : true
ruby2.2 : /abcd*\b/     : true
ruby2.2 : /\b.*abcd*\b/ : true

ruby2.3 : /c.*\b/       : false
ruby2.3 : /abc.*\b/     : false
ruby2.3 : /\b.*abc.*\b/ : false
ruby2.3 : /c.?\b/       : true
ruby2.3 : /abc.?\b/     : true
ruby2.3 : /\b.?abc.?\b/ : true
ruby2.3 : /d*\b/        : true
ruby2.3 : /abcd*\b/     : true
ruby2.3 : /\b.*abcd*\b/ : true

ruby2.4 : /c.*\b/       : false
ruby2.4 : /abc.*\b/     : false
ruby2.4 : /\b.*abc.*\b/ : false
ruby2.4 : /c.?\b/       : true
ruby2.4 : /abc.?\b/     : true
ruby2.4 : /\b.?abc.?\b/ : true
ruby2.4 : /d*\b/        : true
ruby2.4 : /abcd*\b/     : true
ruby2.4 : /\b.*abcd*\b/ : true
~~~

See also:
* https://regex101.com/r/JBzSic/2 (PHP/PCRE, Javascript, Python, Go)
* http://fiddle.re/gkm4ad (Go, Java, Javascript, .Net, Perl, PHP, Python, XRegExp)
* http://java-regex-tester.appspot.com/regex/04925044-ca95-46c6-bec5-329057c04ab2 (Java)



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