regex - Removing spaces between single letters

Question

I have a string that may contain an arbitrary number of single-letters separated by spaces. I am looking for a regex (in Perl) that will remove spaces between all (unknown number) of single letters.

For example:

ab c d should become ab cd

a bcd e f gh should become a bcd ef gh

a b c should become abc

and

abc d should be unchanged (because there are no single letters followed by or preceded by a single space).

Thanks for any ideas.

Answer 1

Your description doesn't really match your examples. It looks to me like you want to remove any space that is (1) preceded by a letter which is not itself preceded by a letter, and (2) followed by a letter which is not itself followed by a letter. Those conditions can be expressed precisely as nested lookarounds:

/(?<=(?<!pL)pL) (?=pL(?!pL))/

tested:

use strict;
use warnings;

use Test::Simple tests => 4;

sub clean {
  (my $x = shift) =~ s/(?<=(?<!pL)pL) (?=pL(?!pL))//g;
  $x;
}

ok(clean('ab c d')        eq 'ab cd');
ok(clean('a bcd e f gh')  eq 'a bcd ef gh');
ok(clean('a b c')         eq 'abc');
ok(clean('ab c d')        eq 'ab cd');

output:

1..4
ok 1
ok 2
ok 3
ok 4

I'm assuming you really meant one space character (U+0020); if you want to match any whitespace, you might want to replace the space with s+.