s/\s\s?(\S)/$1/g
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Close, just drop the \s? and it works.

$ perl -E '$s="2 0 1 2 - 7 - 2 7  9 : 3 7 : 3 1"; $s =~ s/\s\s?(\S)/$1
+/g; say $s'
2012-7-279:37:31

$ perl -E '$s="2 0 1 2 - 7 - 2 7  9 : 3 7 : 3 1"; $s =~ s/\s(\S)/$1/g;
+ say $s'
2012-7-27 9:37:31
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Being lazy, I would abuse the power of regex's and say:

my $string = '2 0 1 2 - 7 - 2 7  9 : 3 7 : 3 1';

$string =~ s/  /x/g;
$string =~ s/ //g;
$string =~ s/x/ /g;

print $string;
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just did it out in steps.. since you want to remove all the spaces I put a spot holder where all the double spaces are supposed to be, then later replaced the 'x' with ' '. perhaps give tr/// a look, that switches out sets but I'm not sure how to switch out spaces with it.

Update: rickoy, welcome to the Monastery!

The specification is a little unclear, but assuming you want to (a) remove all single spaces, and (b) squash all sequences of 2 or more spaces down to a single space:

#! perl
use strict;
use warnings;

my $string = ' 2 0 1 2 - 7 - 2 7  9 : 3 7 : 3 1 ';
#             NB: 2 spaces here ^^

# (a) Remove single spaces
1 while $string =~ s/(^|[^ ])[ ]([^ ]|$)/$1$2/g;

# (b) Squash multiple spaces down to one
$string =~ s/[ ]{2,}/ /g;

print "'", $string, "'\n";
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Outputs:

'2012-7-27 9:37:31'
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HTH,

s/\s(\s?)/$1/g
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Match a single space, and optionally a second space. Capture that second space if it exists. Replace with the capture, which will be either nothing, or the second space.

You could use a negative look-ahead to replace any space that is not followed by a space with nothing. This will break down if there are more than two spaces though.

knoppix@Microknoppix:~$ perl -E '
> $dateStr = q{ 2 0 1 2 - 7 - 2 7  9 : 3 7 : 3 1 };
> $dateStr =~ s{\s(?!\s)}{}g;
> say $dateStr;'
2012-7-27 9:37:31
knoppix@Microknoppix:~$
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Where did your string come from? Strangeness of that sort looks like 16 bit Unicode strings or some such imported in some odd fashion into Perl where the high 0 byte (for an ASCII character) has been replaced by a space. Maybe you would be better to get the conversion right if possible rather than try to fix it up later?