Positive lookahead assertion

[Steve Holden]

tobiah wrote:
>>>Posted via a free Usenet account from http://www.teranews.com
>>
>>Its all about context. If you want to match something but only if it
>>precedes something else, then you follow the regular expression for
>>'something' by the regular expression for 'something else' where
>>`something else` is enclosed by (?=...)
>>
>>The regular expression engine will surreptitiously check that
>>'something else' does indeed follow, before returning any match of
>>'something'.
> 
> 
> 
> How would this differ from just
> 
> re.search('somethingsomething else')
> 
The difference is only significant if your pattern contains groups that 
should be available after the match is complete, or if the match is 
folloed by further match elements. A lookahead assertion does precisely 
that: it looks past the current cursor position and allows an assertion 
about the contents, but without moving the cursor position.

regards
  Steve
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