You could use character classes in your regex

if($string =~ /^R[a-zA-Z]{2}\z/) {
    ...
}
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So that will match 'Rat', 'Rod' and 'Ran' but not 'R4t', 'R0d' or 'RA\n'. For more info on regular expressions see the perlre manpage.
HTH

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broquaint