Extended Notation and Commenting Regular Expressions

News

Perl Regular Expressions

Recommended Links Best Books Perl control structures grep & map
Perl as a command line utility tool Debugging Perl IDE Perl power tools Reimplementation of Unix tools  Extended Notation and Commenting Regular Expressions
Greedy and Non-Greedy Matches Perl Split function Perl HTML Matching Examples Regular Expressions Best Practices index and rindex in Perl Perl tr function

Beautifiers

Perl Warts

 Perl philosophy and history Tips Perl regex history Etc

You want to make your complex regular expressions understandable and maintainable.

Solution

You have four techniques at your disposal: comments outside the pattern, comments inside the pattern with the /x modifier, comments inside the replacement part of s///, and alternate delimiters.

Discussion

The piece of sample code in Example 6.1 uses all four techniques. The initial comment describes the overall intent of the regular expression. For relatively simple patterns, this may be all that is needed. More complex patterns, as in the example, will require more documentation.

#!/usr/bin/perl -p
# resname - change all "foo.bar.com" style names in the input stream
# into "foo.bar.com [204.148.40.9]" (or whatever) instead

use Socket;                 # load inet_addr
s{                          #
    (                       # capture the hostname in $1
        (?:                 # these parens for grouping only
            (?! [-_]  )     # lookahead for neither underscore nor dash
            [\w-] +         # hostname component
            \.              # and the domain dot
        ) +                 # now repeat that whole thing a bunch of times
        [A-Za-z]            # next must be a letter
        [\w-] +             # now trailing domain part
    )                       # end of $1 capture
}{                          # replace with this:
    "$1 " .                 # the original bit, plus a space
           ( ($addr = gethostbyname($1))   # if we get an addr
            ? "[" . inet_ntoa($addr) . "]" #        format it
            : "[???]"                      # else mark dubious
           )
}gex;               # /g for global
                    # /e for execute
                    # /x for nice formatting

For aesthetics, the example uses alternate delimiters. When you split your match or substitution over multiple lines, it helps readability to have matching braces. Another common reason to use alternate delimiters is when your pattern or replacement contains slashes, as in s/\/\//\/..\//g, alternate delimiters makes such patterns easier to read, as in s!//!/../!g or s{//}{/../}g.

The /x modifier makes Perl ignore most whitespace in the pattern (it still counts in a bracketed character class) and treat # characters and their following text as comments. Although useful, this can prove troublesome if you want literal whitespace or # characters in your pattern. If you do want these characters, you'll have to quote them with a backslash, as in the escaped pound signs here:

s/                  # replace
  \#                #   a pound sign
  (\w+)             #   the variable name
  \#                #   another pound sign
/${$1}/xg;          # with the value of the global variable

Remember that comments should explain the text, not just restate the code. Using "$i++ # add one to i" is apt to lose marks in your programming course or get you talked about by your coworkers.

The final technique is /e, which evaluates the replacement portion as a full Perl expression, not just as a (double-quote interpolated) string. The result of running this code is used as the replacement string. Because it is evaluated as code, you can put comments in it. This slows your code down somewhat, but not as much as you'd think (until you write a benchmark on your own, a good idea that will allow you to develop a feel for the efficiency of different constructs). That's because the right-hand side of the substitute is syntax-checked and compiled at compile-time along with the rest of your program. This may be overkill in the case of a simple string replacement, but it is marvelous for more complex cases.

Doubling up the /e to make /ee (or even more, like /eee!) is like the eval "STRING" construct. This allows you to use lexical variables instead of globals in the previous replacement example.

s/                  # replace
\#                  #   a pound sign
(\w+)               #   the variable name
\#                  #   another pound sign
/'$' . $1/xeeg;     # with the value of *any* variable

After a /ee substitution, you can test the $@ variable. It contains any error messages resulting from running your code, because this is real run-time code generation  - unlike /e



Etc

Society

Groupthink : Two Party System as Polyarchy : Corruption of Regulators : Bureaucracies : Understanding Micromanagers and Control Freaks : Toxic Managers :   Harvard Mafia : Diplomatic Communication : Surviving a Bad Performance Review : Insufficient Retirement Funds as Immanent Problem of Neoliberal Regime : PseudoScience : Who Rules America : Neoliberalism  : The Iron Law of Oligarchy : Libertarian Philosophy

Quotes

War and Peace : Skeptical Finance : John Kenneth Galbraith :Talleyrand : Oscar Wilde : Otto Von Bismarck : Keynes : George Carlin : Skeptics : Propaganda  : SE quotes : Language Design and Programming Quotes : Random IT-related quotesSomerset Maugham : Marcus Aurelius : Kurt Vonnegut : Eric Hoffer : Winston Churchill : Napoleon Bonaparte : Ambrose BierceBernard Shaw : Mark Twain Quotes

Bulletin:

Vol 25, No.12 (December, 2013) Rational Fools vs. Efficient Crooks The efficient markets hypothesis : Political Skeptic Bulletin, 2013 : Unemployment Bulletin, 2010 :  Vol 23, No.10 (October, 2011) An observation about corporate security departments : Slightly Skeptical Euromaydan Chronicles, June 2014 : Greenspan legacy bulletin, 2008 : Vol 25, No.10 (October, 2013) Cryptolocker Trojan (Win32/Crilock.A) : Vol 25, No.08 (August, 2013) Cloud providers as intelligence collection hubs : Financial Humor Bulletin, 2010 : Inequality Bulletin, 2009 : Financial Humor Bulletin, 2008 : Copyleft Problems Bulletin, 2004 : Financial Humor Bulletin, 2011 : Energy Bulletin, 2010 : Malware Protection Bulletin, 2010 : Vol 26, No.1 (January, 2013) Object-Oriented Cult : Political Skeptic Bulletin, 2011 : Vol 23, No.11 (November, 2011) Softpanorama classification of sysadmin horror stories : Vol 25, No.05 (May, 2013) Corporate bullshit as a communication method  : Vol 25, No.06 (June, 2013) A Note on the Relationship of Brooks Law and Conway Law

History:

Fifty glorious years (1950-2000): the triumph of the US computer engineering : Donald Knuth : TAoCP and its Influence of Computer Science : Richard Stallman : Linus Torvalds  : Larry Wall  : John K. Ousterhout : CTSS : Multix OS Unix History : Unix shell history : VI editor : History of pipes concept : Solaris : MS DOSProgramming Languages History : PL/1 : Simula 67 : C : History of GCC developmentScripting Languages : Perl history   : OS History : Mail : DNS : SSH : CPU Instruction Sets : SPARC systems 1987-2006 : Norton Commander : Norton Utilities : Norton Ghost : Frontpage history : Malware Defense History : GNU Screen : OSS early history

Classic books:

The Peter Principle : Parkinson Law : 1984 : The Mythical Man-MonthHow to Solve It by George Polya : The Art of Computer Programming : The Elements of Programming Style : The Unix Haterís Handbook : The Jargon file : The True Believer : Programming Pearls : The Good Soldier Svejk : The Power Elite

Most popular humor pages:

Manifest of the Softpanorama IT Slacker Society : Ten Commandments of the IT Slackers Society : Computer Humor Collection : BSD Logo Story : The Cuckoo's Egg : IT Slang : C++ Humor : ARE YOU A BBS ADDICT? : The Perl Purity Test : Object oriented programmers of all nations : Financial Humor : Financial Humor Bulletin, 2008 : Financial Humor Bulletin, 2010 : The Most Comprehensive Collection of Editor-related Humor : Programming Language Humor : Goldman Sachs related humor : Greenspan humor : C Humor : Scripting Humor : Real Programmers Humor : Web Humor : GPL-related Humor : OFM Humor : Politically Incorrect Humor : IDS Humor : "Linux Sucks" Humor : Russian Musical Humor : Best Russian Programmer Humor : Microsoft plans to buy Catholic Church : Richard Stallman Related Humor : Admin Humor : Perl-related Humor : Linus Torvalds Related humor : PseudoScience Related Humor : Networking Humor : Shell Humor : Financial Humor Bulletin, 2011 : Financial Humor Bulletin, 2012 : Financial Humor Bulletin, 2013 : Java Humor : Software Engineering Humor : Sun Solaris Related Humor : Education Humor : IBM Humor : Assembler-related Humor : VIM Humor : Computer Viruses Humor : Bright tomorrow is rescheduled to a day after tomorrow : Classic Computer Humor

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D


Copyright © 1996-2021 by Softpanorama Society. www.softpanorama.org was initially created as a service to the (now defunct) UN Sustainable Development Networking Programme (SDNP) without any remuneration. This document is an industrial compilation designed and created exclusively for educational use and is distributed under the Softpanorama Content License. Original materials copyright belong to respective owners. Quotes are made for educational purposes only in compliance with the fair use doctrine.

FAIR USE NOTICE This site contains copyrighted material the use of which has not always been specifically authorized by the copyright owner. We are making such material available to advance understanding of computer science, IT technology, economic, scientific, and social issues. We believe this constitutes a 'fair use' of any such copyrighted material as provided by section 107 of the US Copyright Law according to which such material can be distributed without profit exclusively for research and educational purposes.

This is a Spartan WHYFF (We Help You For Free) site written by people for whom English is not a native language. Grammar and spelling errors should be expected. The site contain some broken links as it develops like a living tree...

You can use PayPal to to buy a cup of coffee for authors of this site

Disclaimer:

The statements, views and opinions presented on this web page are those of the author (or referenced source) and are not endorsed by, nor do they necessarily reflect, the opinions of the Softpanorama society. We do not warrant the correctness of the information provided or its fitness for any purpose. The site uses AdSense so you need to be aware of Google privacy policy. You you do not want to be tracked by Google please disable Javascript for this site. This site is perfectly usable without Javascript.

Last modified: March, 12, 2019