# Possible regular expression

**URL:** <https://www.ruby-forum.com/t/possible-regular-expression/136747>\
**Category:** Ruby\
**Created:** [May 6, 2008, 3:42am UTC](https://www.ruby-forum.com/t/possible-regular-expression/136747 "2008-05-06T03:42:25Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![jspencer78](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/f14d63/32.png) [@jspencer78](https://www.ruby-forum.com/u/jspencer78)\
**Post date:** [May 6, 2008, 3:42am UTC](https://www.ruby-forum.com/t/possible-regular-expression/136747/1 "2008-05-06T03:42:25Z")

</div>

Ruby’s regular expression engine appears to act incorrectly when given a  
non-greedy match-range of the form {m,n}?

Take this example:

“Age: 21” =~ /Age.{0,60}: ([\w]+)/

This returns 0, as expected and $1 is set to “21”

However:

“Age: 21” =~ /Age.{0,60}?: ([\w]+)/

This returns nil and $1 is set to nil.

I believe the greedy and non-greedy cases should be equivalent in this  
case, but are not.

I’ve included a tarball with two files, one written in perl and the  
other in ruby, performing this match. The perl script acts as expected.

Apologies if this is a known bug that I have been unable to find on  
RubyForge, or if this is expected behavior. If it is the former I would  
appreciate anyone who could point me at the bug listing, and if it is  
the latter, I would appreciate enlightenment on the reason for this  
behavior. In any other case, it would be much appreciated for anyone to  
verify that this behavior is a bug, and I will file it.

Thanks

---

<div class="post-metadata">

**Author:** ![jspencer78](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/f14d63/32.png) [@jspencer78](https://www.ruby-forum.com/u/jspencer78)\
**Post date:** [May 6, 2008, 4:55am UTC](https://www.ruby-forum.com/t/possible-regular-expression/136747/2 "2008-05-06T04:55:37Z")

</div>

On May 5, 7:42 pm, James S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> “Age: 21” =~ /Age.{0,60}?: ([\w]+)/
> 
> This returns nil and $1 is set to nil.

This seems like a bug, given:  
s = “Age: 21”  
s =~ /Age._: (\w+)/ #=\> 0  
s =~ /Age._?: (\w+)/ #=\> 0  
s =~ /Age.{0,60}: (\w+)/ #=\> 0  
s =~ /Age.{0,60}?: (\w+)/ #=\> nil

(Perhaps you were pairing down a real-world testcase; did you know  
that you can simply use \w+ instead of [\w]+ to match one-or-more-word-  
characters? And that \d may be more appropriate, matching only digit  
characters?)

My simple experiments make me believe this is an edge case  
specifically when:  
a) a non-greedy range  
b) that is matching any-char  
c) has a lower-limit of 0  
d) and must match 0 times to succeed.

Here’s my test data, with analysis following.

s = “abbc”  
%w|  
ab{1,9}c ab{1,9}?c  
abb{1,9}c abb{1,9}?c  
abbb{1,9}c abbb{1,9}?c  
ab{0,9}c ab{0,9}?c  
abb{0,9}c abb{0,9}?c  
abbb{0,9}c abbb{0,9}?c  
a.{1,9}c a.{1,9}?c  
ab.{1,9}c ab.{1,9}?c  
abb.{1,9}c abb.{1,9}?c  
a.{0,9}c a.{0,9}?c  
ab.{0,9}c ab.{0,9}?c  
abb.{0,9}c abb.{0,9}?c  
|.each\_with\_index{ |pattern,i|  
regex = Regexp.new( pattern )  
puts “%2i %-15s %s” % [  
i, regex.inspect, (s =~ regex).inspect  
]  
}

#=\> 0 /ab{1,9}c/ 0  
#=\> 1 /ab{1,9}?c/ 0  
#=\> 2 /abb{1,9}c/ 0  
#=\> 3 /abb{1,9}?c/ 0  
#=\> 4 /abbb{1,9}c/ nil  
#=\> 5 /abbb{1,9}?c/ nil  
#=\> 6 /ab{0,9}c/ 0  
#=\> 7 /ab{0,9}?c/ 0  
#=\> 8 /abb{0,9}c/ 0  
#=\> 9 /abb{0,9}?c/ 0  
#=\> 10 /abbb{0,9}c/ 0  
#=\> 11 /abbb{0,9}?c/ 0  
#=\> 12 /a.{1,9}c/ 0  
#=\> 13 /a.{1,9}?c/ 0  
#=\> 14 /ab.{1,9}c/ 0  
#=\> 15 /ab.{1,9}?c/ 0  
#=\> 16 /abb.{1,9}c/ nil  
#=\> 17 /abb.{1,9}?c/ nil  
#=\> 18 /a.{0,9}c/ 0  
#=\> 19 /a.{0,9}?c/ 0  
#=\> 20 /ab.{0,9}c/ 0  
#=\> 21 /ab.{0,9}?c/ 0  
#=\> 22 /abb.{0,9}c/ 0  
#=\> 23 /abb.{0,9}?c/ nil

In the above, we would expect patterns 4, 5, 16 and 17 to fail, but  
not 23.

Notable is that pattern #15 succeeds (showing that a non-greedy range  
matching any-char can match a lower-limit number of times) and that  
pattern #11 succeeds (showing that a non-greedy range matching a  
specific char can match zero number of times).

---

<div class="post-metadata">

**Author:** ![jspencer78](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/f14d63/32.png) [@jspencer78](https://www.ruby-forum.com/u/jspencer78)\
**Post date:** [May 6, 2008, 5:00am UTC](https://www.ruby-forum.com/t/possible-regular-expression/136747/3 "2008-05-06T05:00:53Z")

</div>

On May 5, 7:42 pm, James S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Ruby’s regular expression engine appears to act incorrectly when given a  
> non-greedy match-range of the form {m,n}?

I forgot to note, in my previous reply, that my test results are  
against 1.8.6:  
ruby 1.8.6 (2007-09-24 patchlevel 111) [i686-darwin9.1.0]

Ruby v1.9 (using a different regexp engine, “Oniguruma”) does not  
suffer from the same problem.

---

<div class="post-metadata">

**Author:** ![jspencer78](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/f14d63/32.png) [@jspencer78](https://www.ruby-forum.com/u/jspencer78)\
**Post date:** [May 6, 2008, 5:20am UTC](https://www.ruby-forum.com/t/possible-regular-expression/136747/4 "2008-05-06T05:20:45Z")

</div>

On May 5, 8:59 pm, Phrogz [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> suffer from the same problem.  
> Rubinius and JRuby don’t seem to suffer from it either.

Chris

---

<div class="post-metadata">

**Author:** ![jspencer78](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/f14d63/32.png) [@jspencer78](https://www.ruby-forum.com/u/jspencer78)\
**Post date:** [May 6, 2008, 6:33am UTC](https://www.ruby-forum.com/t/possible-regular-expression/136747/5 "2008-05-06T06:33:35Z")

</div>

Thank you Gavin and Chris for your verification. Gavin, you are right  
that it is pared down from a real problem where a character class and  
alphanumerics were necessary, thank you for your much better examples.  
I’ll file a bug report against 1.8.6.

-James