This will probably do what you are asking for: 

$text =~ s[\b([Cc]at)\b][<span class="topic">$1</a>]g;

[download]

but you also probably want </span> rather than </a> as requested :)

Examine what is said, not who speaks.
Silence betokens consent.
Love the truth but pardon error.

In reply to Re: case preservation in regexp by BrowserUk
in thread case preservation in regexp by water

Title:
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