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Regular expressions are one of the most powerful tools in the arsenal of any *NIX programmer. This article offers insights into what they are, how to go about constructing them, and how to add them to your Perl, PHP and JavaScript programs.

  1. So What's A $#!%% Regular Expression, Anyway?!
  2. Ranging Far And Wide...
  3. How To Say "Ummmm...." In Three Different Languages
By: Vikram Vaswani and Harish Kamath, (c) Melonfire
Rating: starstarstarstarstar / 5
April 12, 2000

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Now that we've got all that out of the way, let's take a closer look at some examples of how regular expressions are used in Perl, PHP and JavaScript. In Perl, for example, you can perform some pretty advanced pattern matching using both the rules you've already learnt, and some Perl-specific additions.

A pattern-matching command in Perl usually looks like this:

operator / regular-expression / string-to-replace / modifiers
Let's take a closer look at each of these components.

The operator can either be an "m" or an "s", depending on the purpose of the regular expression -"m" is used for "match" operations only, while "s" is used for "substitution" operations.

The regular expression is the pattern that is to be matched. This pattern can be constructed using a variety of characters, meta-characters and pattern anchors.

The string to replace is...well, the string to be replaced in a find-and-replace operation. Yeah, every once in a while, we slip you an easy one.

Finally, the modifiers are used to control the manner in which a particular regex is applied. There are a whole bunch of modifiers, some of them with pretty exotic names; unfortunately, none of them are single, or interested in going out to dinner with you.

So, the statement

would replace the first occurrence of the word "love" with "lust". And if you wanted to perform a global search-and-replace operation, you'd use the "g" modifier, like this

And they say romance is dead!

You can also use case-insensitive pattern matching - simply add the "i" modifier, as in the following example, and watch in awe as Perl matches "jewel", "Jewel" and "JEWEL".

In Perl, all interaction with regular expressions takes place via an equality operator, represented by =~; this is used as follows.

$flag =~ m/abc/
$flag returns true if $flag contains "abc"

$flag =~ s/abc/ABC/
replaces abc in the variable $flag with ABC And here's an example of a simple Perl program which asks for your email address, and compares it with a regex to verify whether or not it's in the correct format.

#!/usr/bin/perl # get input print "So what's your email address, anyway?n"; $email = <STDIN>; chomp($email); # match and display result if($email =~ /^([a-zA-Z0-9_-])+@([a-zA-Z0-9_-])+(.[a-zA-Z0-9_-])+/) { print("Ummmmm....that sounds good!n"); } else { print("Hey - who do you think you're kidding?n"); }
As you can see, the most important part of this program is the regular expression - it's been dissected below:

The first part

matches the username part of the email address - this could be either a number, a character, or a combination of both.

This is followed by an @ symbol, which is followed by the domain part of the address; this could again include letters or numbers, and uses a period as a delimiter - not our usage of an "escaped" period and the "+" meta-character to represent these conditions in the second half of the expression

Obviously, this is simply an illustrative example - if you're planning to use it on your Web site, you need to refine it a bit. For example, the script above won't accept email addresses of the form firstname.lastname@somedomain.com - although such addresses are also pretty common on the Web. You have been warned!

If you prefer PHP to Perl, you need to use the ereg() function for all pattern matching operations,this usually takes the format

ereg(pattern, string)
where "pattern" is the pattern to be matched, and "string" is the character string to be searched for the pattern. The next example should illustrate this a little more clearly:

<?php if (ereg("^([a-zA-Z0-9_-])+@([a-zA-Z0-9_-])+(.[a-zA-Z0-9_-])+",$email)) { echo "Ummmmm....that sounds good!"; } else { echo "Hey - who do you think you're kidding?"; } ?>
And finally, JavaScript. JavaScript 1.2 comes with a powerful RegExp() object, which can be used to match patterns in strings and variables. The important thing here is the test() method, which searches for a pattern in a string or variable, and returns either true or false - its illustrated in the example below.

<html> <head> <script language="Javascript1.2"> <!-- start hiding function verifyAddress(obj) { // obtain form value into variable var email = obj.email.value; // define regex var pattern = /^([a-zA-Z0-9_-])+@([a-zA-Z0-9_-])+(.[a-zA-Z0-9_-])+/; // test for pattern flag = pattern.test(email); if(flag) { alert("Ummmmm....that sounds good!"); return true; } else { alert("Hey - who do you think you're kidding?"); return false; } } // stop hiding --> </script> </head> <body> <form onSubmit="return verifyAddress(this);"> <input name="email" type="text"> <input type="submit"> </form> </body> </html>
Obviously, there's a whole lot more that you can do with regular expressions - checking email addresses is just the tip of the iceberg. You can use regular expressions to validate phone numbers, currency figures, Web site URLs, and a whole lot more - all you need is a little bit of creativity and patience, a few slices of leftover pizza...and a therapist who cares.

>>> More Site Administration Articles          >>> More By Vikram Vaswani and Harish Kamath, (c) Melonfire

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