if($string =~ /^R[a-zA-Z]{2}\z/) {
    ...
}
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_________
broquaint

while (<DATA>) {
    chomp;
    print if substr($_,0,1) eq 'R' && length == 3;
}
 
__DATA__
raa
Raa
Raab
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Update: Rereading your question, my solution only applies if you read 'letter' as 'any character'.

-- Hofmator

Simple regexp.. if ($string=~ /^R..$/i) { #bleah }. The i at the end makes it case-insensitive, if you want it case sensitive (ie. matches 'Ray' but not 'ray') then remove it.

Update: Thanks Juerd for the correction below, although realised we're both wrong!

The following will match any three letter string, starting with a 'r' and optionally ending with a newline... if ($string =~ /^R[A-Z][A-Z]\n?$/i) { # bLEEEA }

Again this is case insensitive, to make it sensitive now though you need to change the A-Z bits into which case you want.

/^R..$/

That will match ray but also ray\n, but not ra\n.

/^R..\z/s
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\z is end of string, /s makes . match \n.

- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

If you've got the /s switch on though this'll happily match R\n\n\n, won't it?

Update: Oops, did wrong number of \n's. R\n\n does seem to pass this match, unless the code below is wrong..

#!/usr/bin/perl -w

use strict;

my @tests = (
  "R\n\n",        # Passes
  "Roo",          # Passes
  "roo",          # Fails
  "Roo\n",        # Fails
);

foreach (@tests) {
  if (/^R..\z$/s) {
    print "$_ passes\n";
  } else {
    print "$_ fails\n";
  }
}
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