Is this what you are looking for? 
$ perl -e '$_ = "(b)."; /(\(?=a|b|c\))/ && do { print "match!\n"; };'
match!

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The outer parenthesis captures the match and the inner should be escaped!
UPDATE
Never mind, didn't pay attention to the atomic grouping!

In reply to Re: Why does this simple grouping regex not match? by umasuresh
in thread Why does this simple grouping regex not match? by december

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