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Regular Expression - Regex

A regular expression is a pattern describing a certain amount of text. It is also called regex.

 

A way of describing a pattern in text - for example:

• all the words that begin with the letter
• every 10-digit phone number
• every sentence with two commas in it, and no capital letter Q

Check for a phone number pattern:

 /^([1]-)? [0-9]{3}- [0-9]{3}- [0-9]{4} $/i   

• slashes (/) at the start and end are the delimiters that signal where the pattern starts and ends.
• caret (^) indicates the beginning of the string we want to check
• ([1]-)? is asking “is there a ’1′ followed by a hyphen. The ? question mark at the end makes this portion optional. In other words, if it’s not there — meaning that it’s not an 800 number — it won’t immediately assume that the phone number is invalid.
• [0-9]{3}- is looking for ONLY numbers [0-9] and there must be three {3} of them. Now, the added hyphen after the {3}- makes it so that hyphens are required in the phone number.
• [0-9]{3}- is doing the same thing — making sure there are three numbers and a hyphen. Now, if you will notice, the last of these ends with {4}. As you my have guessed, this requires there to be four digits in the last part of the number.
• The $ at end signifies that this is the end of our string.
• i after the final slash (/i) is a switch that tells regex to perform a case-insensitive search. In other words “aaa” would be considered the same as “AAA”. This isn’t too important when validating a phone number, but you should definitely include it when validating an email address.

Check for email pattern:

/^[_a-z0-9-]+(.[_a-z0-9-]+)*@[a-z0-9-]+(.[a-z0-9-]+)*(.[a-z]{2,3})$/i  

• amount of letters or numbers
• followed by the @ symbol
• followed by any letters or numbers (the domain name)
• followed by a .
• and then ending with any combination of 2 or 3 letters

Find a Phrase in a Phrase

<?php
// The "i" after the pattern delimiter indicates a case-insensitive search
if (preg_match("/php/i", "PHP is the web scripting language of choice.")) {
echo "A match was found.";
 } else {
 echo "A match was not found.";
 }
 ?>

Find a Word in a Phrase

<?php
/* The \b in the pattern indicates a word boundary, so only the distinct word "web" is matched, 
and not a word partial like "webbing" or "cobweb" */
 if (preg_match("/\bweb\b/i", "PHP is the web scripting language of choice.")) {
 echo "A match was found.";
 } else {
 echo "A match was not found.";
 }
 

Using named subpattern

<?php
     $str = 'foobar: 2008';
     preg_match('/(?P<name>\w+): (?P<digit>\d+)/', $str, $matches);
     /* This also works in PHP 5.2.2 (PCRE 7.0) and later, however 
     * the above form is recommended for backwards compatibility */
     // preg_match('/(?<name>\w+): (?<digit>\d+)/', $str, $matches);
     
     print_r($matches);
     
     ?>

The above example will output:

Array  (      
		[0] => foobar: 2008      
		[name] => foobar      
		[1] => foobar      
		[digit] => 2008      
		[2] => 2008  
)  

Learning Regex

• Meta-characters
• Regular Expression Tutorial - Learn How to Use and Get The Most out of Regular Expressions
• Web Cheat Sheet - Using Regular Expressions with PHP
• Wikipedia - Regular_expression

Links

• Regular Expression Basic Syntax Reference
• Sample Regular Expressions
• http://www.regular-expressions.info/regexbuddy/email.html
• Advanced regexps
• PHP preg_match Regular Expression (Regexp) Tester

Regular Expression	Will match...
foo	The string "foo"
^foo	"foo" at the start of a string
foo$	"foo" at the end of a string
^foo$	"foo" when it is alone on a string
[abc]	a, b, or c
[a-z]	Any lowercase letter
[^A-Z]	Any character that is not a uppercase letter
(gif|jpg)	Matches either "gif" or "jpeg"
[a-z]+	One or more lowercase letters
[0-9\.\-]	Аny number, dot, or minus sign
^[a-zA-Z0-9_]{1,}$	Any word of at least one letter, number or _
([wx])([yz])	wy, wz, xy, or xz
[^A-Za-z0-9]	Any symbol (not a number or a letter)
([A-Z]{3}|[0-9]{4})	Matches three letters or four numbers

 

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Page last updated: May 31, 2012 10:46 AM