What is wrong with this substitution?

Question

#!/usr/bin/perl

use strict;
use warnings;

my $s = "sad day
 Good day
 May be Bad Day 
 ";

$s =~ s/\w+ \w+/_/gm;

print $s;

I am trying to substitute all spaces between words with _, but it is not working. What is wrong with that?

Answer 1

The substitution replaces an entire word (\w+) then a space then an other word by an underscore.

There is really no need to replace (or capture for what matters) those words

$a=~s/\b +\b/_/gm;

will replace a word break ( \b, a zero-width transition between a word and a non word) followed by one or more spaces followed by an other word break, by an underscore. Using \b ensures that you don't replace a space after or before a new line.