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$ perl -Mre=debug -le " q/a a/ =~ /(.\b)\1/ "
Compiling REx "(.\b)\1"
...
                                  failed...
Match failed
Freeing REx: "(.\b)\1"

$ perl -Mre=debug -le " q/aa/ =~ /a\b/ "
Compiling REx "a\b"
Final program:
...
   2 <aa> <>                 |  4:END(0)
Match successful!
Freeing REx: "a\b"

     $x = "Housecat catenates house and cat";
    $x =~ /\bcat/; # matches cat in 'catenates'
    $x =~ /cat\b/; # matches cat in 'housecat'
    $x =~ /\bcat\b/; # matches 'cat' at end of string