The regexp you're using will get a false hit on thistle.

Try 

my $search = "this";
my $data = "This is the test text for THIS routine.";
my $bs = "<b>";
my $be = "</b>";
my $count = $data =~ s/\b($search)\b/$bs\1$be/gi;

[download]
The backreference (\1) preserves case.


Update: Kanji is right. Use $1 instead of \1.

In reply to Re: Matching problems by dws
in thread Matching problems by Anonymous Monk

Title:
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