Positive lookahead assertion

[tobiah]

(?=...)
Positive lookahead assertion. This succeeds if the contained 
regular expression, represented here by ..., successfully 
matches at the current location, and fails otherwise. 
But, once the contained expression has been tried, the 
matching engine doesn't advance at all; the rest of the 
pattern is tried right where the assertion started.

I am unable to wrap my mind around this sentence.  Could
someone give me an example of how this works, and why
it would be useful?

Thanks,

Toby

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