Question

I came across this regular expression in the jQuery source code:

...
rmozilla = /(mozilla)(?:.*? rv:([\w.]+))?/,
...

I was wondering why it was rather complicated. I’m especially interested in the reason behind the second part:

(?:.*? rv:([\w.]+))?

I did some research but I could not figure out what this part of the regular expression adds.

(?:)      to match but not capture
.*?       any amount of any character
 rv:      something literal
([\w.]+)  one or more word characters or a dot
?         appear 0 or 1 time

Particularly, that last ? doesn’t make much sense to me. The whole second part matches if there is or is not a substring as defined by that second part. With some trial and error the regular expression does not seem to differ from just:

/(mozilla)/

Could someone shed some light on what the second part of the regular expression is supposed to do? What does it constrain; what string fails that passes /(mozilla)/ or the other way round?

Answer 1

The two regexes would match the same strings, but would store different information in their capturing groups.

for the string: mozilla asdf rv:sadf

/(mozilla)(?:.*? rv:([\w.]+))?/
$0 = 'mozilla asdf rv:sadf'
$1 = 'mozilla'
$2 = 'sadf'

/(mozilla)/
$0 = 'mozilla'
$1 = 'mozilla'
$2 = ''