perlpal's example was $string =~ s/ {1,}/_/g; which matches a space character while your example $string =~ s/\s+/\_/g; uses the whitespace character class (\s) which matches "\t", "\r", "\n" and "\f" as well as the space character.    In general, the tr/// operator is more efficient then the s/// operator so I try to use it when appropriate.