Regular Expressions

Introduction

PHP regular expressions use the "Perl Compatible Regular Expressions" (PCRE) syntax. You can read more about PCRE in this Wikipedia article.

What this PCRE compatibility means for you, is that if you learn the PHP regular expression syntax, you will also understand regular expressions in a lot of other systems such as JavaScript, ColdFusion, etc. And if you already know about regular expressions in these other systems, you should be comfortable quickly with PHP's regular expressions.

A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in the pattern, and match the corresponding characters in the subject. As a trivial example, the pattern The quick brown fox matches a portion of a subject string that is, not surprisingly, The quick brown fox.

Delimiters

A regular expression must be enclosed in delimiters. A very commonly-used delimiter for regular expressions is the forward slash (/).

Here are some samples of regular expressions:

• /mongol hoards/
• /^\d{3}(\s|-)?\d{2}(\s|-)?\d{4}$/    (a Social Security Number)
• /^\(?\d{3}\)?(\s|\.|-)?\d{3}(\s|-|\.)?\d{4}$/    (a simple telephone number)
• /^\d{1,2}(\-|\/|\.)\d{1,2}\1\d{4}$/    (a date in the format mm/dd/yyyy)

Meta-Characters

Meta-characters in a regular expression do not stand for themselves, but rather are interpreted in a special way.

Some of the most common meta-characters are:

Escape Sequences

The backslash character has several uses. First, if it is followed by a non-alphanumeric character, it takes away any special meaning that character may have. This use of backslash as an escape character applies both inside and outside character classes.

For example, if you want to match a * character, you write \* in the pattern. This rule applies whether or not the following character would otherwise be interpreted as a meta-character, so it is always safe to precede a non-alphanumeric character with \ to specify that it stands for itself. This consideration also means that if you want to match a backslash, you put \\ into the pattern.

In PHP regular expressions, you need to remember that quoted strings give special meaning to the backslash. So if you need to match \ with a regular expression \\ in a PHP string, you must put either "\\\\" or '\\\\' into the pattern.

You can also use the backslash in regular expressions to encode character sequences in a compact format. Here are some of the most commonly-used character-sequence escape sequences:

Testing Text

You can use regular expressions to test whether a pattern is in a character (text) string.

One very handy thing that you can do with this text testing is to validate form input before you change the fields in a database table, for instance.

In fact, the e-Handout "PHP Form Validation" has at least one regular expression in its class definition for the FormValidator object. Look for the sub-section "isEmailAddress()", and notice the value that is assigned to the variable $pattern. That value is a regular expression which tests for a basic e-mail address format.

The regular expression that is assigned to the variable $pattern is this:

	"/^([a-zA-Z0-9])+([\.a-zA-Z0-9_-])*@([a-zA-Z0-9_-])+(\.[a-zA-Z0-9_-]+)+/"

The sample code from the e-Handout section "isEmailAddress()" "tests" the text value that came to this page, probably from an HTML form. These lines of code do the testing (starting with the call to the function preg_match()):

	if(preg_match($pattern, $value))
	{
		return true;
	}
	else
	{
		$this->_errorList[] = array("field" => $field,
		"value" => $value, "msg" => $msg);
		return false;
	}

The various portions in the pattern in the above regular expression are explained here:

Please note that the built-in PHP function preg_match() is only one of a set of functions which use regular expressions. You can find one list of these functions here at the PHP.net site.

More Patterns

You can find regular expressions at many online sites, which can accomplish many specific tests.

Following is a table of some of the more common regular expressions which you may be able to use in your PHP pages: