An excerpt from chromatic's Modern Perl book about clean regexes

clean-regex.pl

# The /x modifier allows you to embed additional whitespace and comments within patterns. 
# With this modifier in effect, the regex engine ignores whitespace and comments. 
# The results are often much more readable.

# This regex isn't simple, but comments and whitespace improve its readability.
# Even if you compose regexes together from compiled fragments, the /x modifier 
# can still improve your code.

my $attr_re = qr{
    \A                    # start of line
     (?:
      [;\n\s]*            # spaces and semicolons
      (?:/\*.*?\*/)?      # C comments
    )*

    ATTR

    \s+
    (   U?INTVAL
      | FLOATVAL
      | STRING\s+\*
    )
}x;