use strict;
my $a='ABCAAAABBBCCCCCAAABBBCCC';
my $b='ASRGRTGRT89579843rrrrrr';
print '$a matches',$/ if $a =~ /^[ABC]+$/;
print '$b matches',$/ if $b =~ /^[ABC]+$/;

[download]

Update: The regex is anchored at the beginning and end of the string, and requires 1 or more of character class [ABC]. 

perl -MYAPE::Regex::Explain -e 'print YAPE::Regex::Explain->new(qr/^[A
+BC]+$/)->explain;'
The regular expression:

(?-imsx:^[ABC]+$)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  [ABC]+                   any character of: 'A', 'B', 'C' (1 or more
                           times (matching the most amount possible))
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]

Update2:

Of course, this is not at all efficient, checking every character in the string to ensure it is one that you want. The responses below are much more efficient.

In reply to Re: Match only certain characters? by keszler
in thread Match only certain characters? by Anonymous Monk

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