Not the prettiest, but this should work: 

  $n =~ s/(?:^|([ ]))$a(?:([ ])|$)/($1 ? $1 : "") . $b . ($2 ? $2 : ""
+)/ie;

[download]
Warning: Code is untested; note the use of "should" vs. "will".

In reply to Re: regexp help -- word boundaries by northwind
in thread regexp help -- word boundaries by water

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