mystery regexp

[Steve Throckmorton]

I found this regexp in the docs for the OptionParser class, and I don't
understand it. Google hasn't helped, nor has the Pickaxe, so I thought
I would ask here. The purpose of the code is to ensure that a file
extension submitted by the user begins with a period (a dot).

file_ext.sub!(/\A\.(?=.)/, ".")

I understand all except (?=.) As far as I can tell the regexp works
fine without it. I can delete the dot inside the parens, and the code
still runs. I can change the = to a - and the code runs, but if I
substitute a letter or a * instead, it doesn't.

Someone please clue me in here. What is this thing?

 

[gregarican]

Check out http://www.zenspider.com/Languages/Ruby/QuickRef.html#11 for
details on this portion of the regex. It seems to involve look-ahead
capability.

HTH.

 

[Steve Throckmorton]

There ya go. Thanks.

Check out http://www.zenspider.com/Languages/Ruby/QuickRef.html#11 for
details on this portion of the regex. It seems to involve look-ahead
capability.

HTH.

 

[Jason Sweat]

I found this regexp in the docs for the OptionParser class, and I don't
understand it. Google hasn't helped, nor has the Pickaxe, so I thought
I would ask here. The purpose of the code is to ensure that a file
extension submitted by the user begins with a period (a dot).

file_ext.sub!(/\A\.(?=.)/, ".")

I understand all except (?=.) As far as I can tell the regexp works
fine without it. I can delete the dot inside the parens, and the code
still runs. I can change the = to a - and the code runs, but if I
substitute a letter or a * instead, it doesn't.

Someone please clue me in here. What is this thing?

\A start of subject (independent of multiline mode)

(?= is a positive look ahead assetion

so \A the begining of the input \. is a literal . (and there must be
something more than that afterwords.

$ irb=> #<MatchData:0x409866a8>

 

[James Edward Gray II]

file_ext.sub!(/\A\.(?=.)/, ".")

I understand all except (?=.)

That's a look-ahead assertion, ensuring there is at least one
character following the period. Observe:
=> nil

Hope that helps.

James Edward Gray II