s/OLD\b/NEW/g;
[download]

Well, sort of ;) Take these examples:

s/\bold\b/new/gi; 
#  1: replaces all instances of the string, e.g. "old", 
#  "OLD", "Old", "OlD" on either side with "new".

s/\bold\b/new/g; 
#  2: replaces "old" with "new" -- case sensitive here.

s/\bOLD\b/NEW/g; 
#  3: replaces "OLD" with "NEW"

s/\bOLD\b/NEW/gi; 
#  4: replaces same variants of "OLD" as in 1 with "NEW"
[download]

So, to finally answer your question: yes, but I think you might need that other "\b".

Something that might help you a lot, as it did for me, is YAPE::Regex and YAPE::Regex::Explain by our very own japhy. They can be downloaded off the CPAN. For example:

#!/usr/bin/perl

use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(qr/\bold\b/i)->explain;
[download]

gives the following explanation.

The regular expression:

(?i-msx:\bold\b)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?i-msx:                 group, but do not capture (case-insensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  old                      'old'
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

--
Allolex

Add a boundary to the beginning and that should do the trick (watch out for words like FOLD, TOLD, etc). Also note that you don't need to write a full Perl script to make the changes to the files in question. You can do this with a one-liner:

perl -pi -e's/\bOLD\b/NEW/g' *.txt
[download]

And if you are paranoid and want to keep the orignals:

perl -pi.bak -e's/\bOLD\b/NEW/g' *.txt
[download]

will preserve them with a .bak "extension" added to the end of the filename. (For example, foo.txt becomes foo.txt.bak)

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)