From: PerlFAQ Server on
This is an excerpt from the latest version perlfaq6.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

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6.1: How can I hope to use regular expressions without creating illegible and unmaintainable code?

Three techniques can make regular expressions maintainable and
understandable.

Comments Outside the Regex
Describe what you're doing and how you're doing it, using normal
Perl comments.

# turn the line into the first word, a colon, and the
# number of characters on the rest of the line
s/^(\w+)(.*)/ lc($1) . ":" . length($2) /meg;

Comments Inside the Regex
The "/x" modifier causes whitespace to be ignored in a regex pattern
(except in a character class), and also allows you to use normal
comments there, too. As you can imagine, whitespace and comments
help a lot.

"/x" lets you turn this:

s{<(?:[^>'"]*|".*?"|'.*?')+>}{}gs;

into this:

s{ < # opening angle bracket
(?: # Non-backreffing grouping paren
[^>'"] * # 0 or more things that are neither > nor ' nor "
| # or else
".*?" # a section between double quotes (stingy match)
| # or else
'.*?' # a section between single quotes (stingy match)
) + # all occurring one or more times
> # closing angle bracket
}{}gsx; # replace with nothing, i.e. delete

It's still not quite so clear as prose, but it is very useful for
describing the meaning of each part of the pattern.

Different Delimiters
While we normally think of patterns as being delimited with "/"
characters, they can be delimited by almost any character. perlre
describes this. For example, the "s///" above uses braces as
delimiters. Selecting another delimiter can avoid quoting the
delimiter within the pattern:

s/\/usr\/local/\/usr\/share/g; # bad delimiter choice
s#/usr/local#/usr/share#g; # better



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