Elsewhere I saw a question asking about matching alternating case strings. Here's the regex, and a test script.

m/^([a-z]([A-Z][a-z])*[A-Z]?|[A-Z]([a-z][A-Z])*[a-z]?)$/
[download]

It looks a bit clunky, but it works.

Here's the test script:

#!/usr/bin/env perl

# Test for alternating case regex

use strict;
use warnings;

# Generate all lower/upper combos up to 5 characters with the characte
+rs A-E
my @test_strings = qw(
    a A
    ab aB Ab AB
    abc aBc Abc ABc abC aBC AbC ABC
    abcd aBcd Abcd ABcd abCd aBCd AbCd ABCd
    abcD aBcD AbcD ABcD abCD aBCD AbCD ABCD
    abcde aBcde Abcde ABcde abCde aBCde AbCde ABCde
    abcDe aBcDe AbcDe ABcDe abCDe aBCDe AbCDe ABCDe
    abcdE aBcdE AbcdE ABcdE abCdE aBCdE AbCdE ABCdE
    abcDE aBcDE AbcDE ABcDE abCDE aBCDE AbCDE ABCDE
);

# test all strings, save them in a hash for sorting later
my %results;
for my $test (@test_strings) {
    my $result = $test =~ m/^([a-z]([A-Z][a-z])*[A-Z]?|[A-Z]([a-z][A-Z
+])*[a-z]?)$/;
    $results{$test} = $result;
}

# print the results, sorted by result
for my $key (sort by_result_or_length_or_alpha keys %results) {
    printf "%10s  %d\n", $key, $results{$key};
}

exit;

sub by_result_or_length_or_alpha {
    ($results{$b} <=> $results{$a})
    or
    (length($a) <=> length($b))
    or
    ($b cmp $a)
}
[download]

Here's the output:

         a  1
         A  1
        aB  1
        Ab  1
       aBc  1
       AbC  1
      aBcD  1
      AbCd  1
     aBcDe  1
     AbCdE  1
        ab  0
        AB  0
       abc  0
       abC  0
       aBC  0
       Abc  0
       ABc  0
       ABC  0
      abcd  0
      abcD  0
      abCd  0
      abCD  0
      aBcd  0
      aBCd  0
      aBCD  0
      Abcd  0
      AbcD  0
      AbCD  0
      ABcd  0
      ABcD  0
      ABCd  0
      ABCD  0
     abcde  0
     abcdE  0
     abcDe  0
     abcDE  0
     abCde  0
     abCdE  0
     abCDe  0
     abCDE  0
     aBcde  0
     aBcdE  0
     aBcDE  0
     aBCde  0
     aBCdE  0
     aBCDe  0
     aBCDE  0
     Abcde  0
     AbcdE  0
     AbcDe  0
     AbcDE  0
     AbCde  0
     AbCDe  0
     AbCDE  0
     ABcde  0
     ABcdE  0
     ABcDe  0
     ABcDE  0
     ABCde  0
     ABCdE  0
     ABCDe  0
     ABCDE  0
[download]

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