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"

##</code><code>##

#!/usr/bin/perl

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

##</code><code>##

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
----------------------------------------------------------------------