# Regular expression: Is this a bug or feature?

**URL:** <https://www.ruby-forum.com/t/regular-expression-is-this-a-bug-or-feature/102662>\
**Category:** Ruby\
**Created:** [January 23, 2007, 9:53am UTC](https://www.ruby-forum.com/t/regular-expression-is-this-a-bug-or-feature/102662 "2007-01-23T09:53:22Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![mkrauskopf](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c37758/32.png) [@mkrauskopf](https://www.ruby-forum.com/u/mkrauskopf)\
**Post date:** [January 23, 2007, 9:53am UTC](https://www.ruby-forum.com/t/regular-expression-is-this-a-bug-or-feature/102662/1 "2007-01-23T09:53:22Z")

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Hi ruby experts!

Is this intended behaviour?

irb(main):001:0\> s1=‘a=1’  
=\> “a=1”  
irb(main):002:0\> s2=‘b=1’  
=\> “b=1”  
irb(main):003:0\> s1 =~ /a|b=(.)/  
=\> 0 \<------ expression matches  
irb(main):004:0\> $1  
=\> nil \<------ but where is argument?  
irb(main):005:0\> s2 =~ /a|b=(.)/  
=\> 0 \<------ expression matches  
irb(main):006:0\> $1  
=\> “1” \<------ this has been expected  
irb(main):007:0\> s1 =~ /(a|b)=(.)/  
=\> 0 \<------ expression matches  
irb(main):012:0\> $2  
=\> “1” \<------ this has been expected

Tested on ruby 1.8.2 (2004-12-22) [i686-linux]

Thanks for your help in advance  
Martin.

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**Author:** ![mkrauskopf](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c37758/32.png) [@mkrauskopf](https://www.ruby-forum.com/u/mkrauskopf)\
**Post date:** [January 23, 2007, 5:09pm UTC](https://www.ruby-forum.com/t/regular-expression-is-this-a-bug-or-feature/102662/2 "2007-01-23T17:09:31Z")

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Martin K. wrote:

> irb(main):003:0\> s1 =~ /a|b=(.)/  
> =\> 0 \<------ expression matches  
> irb(main):004:0\> $1  
> =\> nil \<------ but where is argument?

I assume this is the one you need explanation for? I think you simply  
misinterpret the regexp. /a|b=(.)/ is a union between the two regexp /a/  
and /b=(.)/. So in this case it matches only the first one which has no  
bindings. The regexp you are probably looking for would be  
/(?:a|b)=(.)/. Try that.

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<div class="post-metadata">

**Author:** ![mkrauskopf](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/c37758/32.png) [@mkrauskopf](https://www.ruby-forum.com/u/mkrauskopf)\
**Post date:** [September 25, 2007, 10:52pm UTC](https://www.ruby-forum.com/t/regular-expression-is-this-a-bug-or-feature/102662/3 "2007-09-25T22:52:29Z")

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On 1/23/07, Martin K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Hi ruby experts!
> 
> Is this intended behaviour?
> 
> irb(main):001:0\> s1=‘a=1’  
> =\> “a=1”  
> irb(main):002:0\> s2=‘b=1’  
> =\> “b=1”

Call this part 1:

> irb(main):003:0\> s1 =~ /a|b=(.)/  
> =\> 0 \<------ expression matches  
> irb(main):004:0\> $1  
> =\> nil \<------ but where is argument?

Part 2:

> irb(main):005:0\> s2 =~ /a|b=(.)/  
> =\> 0 \<------ expression matches  
> irb(main):006:0\> $1  
> =\> “1” \<------ this has been expected

Part 3:

> irb(main):007:0\> s1 =~ /(a|b)=(.)/  
> =\> 0 \<------ expression matches  
> irb(main):012:0\> $2  
> =\> “1” \<------ this has been expected

I’m not sure why you think it might be a bug. The ‘|’ operator just  
binds very loosely, so you have to group the “a|b” in parens. Note  
that in part 1, The bit that matches is the left side of the ‘|’,  
namely ‘a’ (no parens), so there are no captures. In part 2, the  
right side (‘b=(.)’) matches, so there’s 1 capture. In part 3, it  
matches the whole thing (‘(a|b)=(.)’), so there are 2 captures.

Does this make sense?

Note that if you only want 1 capture, you can also use the shy  
grouping operator (?:…), so:

s1 =~ /(?:a|b)=(.)/

[$1, $2] #=\> [“1”, nil]