Parentheses matter. You didn't capture a $1. 
print "<span class='topic'>$1</span>" if /\b(cat)\b/i;

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If only the first letter should be flexible, 
print "<span class='topic'>$1</span>" if /\b([Cc]at)\b/;

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--
[ e d @ h a l l e y . c c ]

In reply to Re^2: case preservation in regexp by halley
in thread case preservation in regexp by water

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