📖 PHP Regular Expressions

A Regular Expression is a text string that defines a character pattern. It is used to validate form input to insure it matches a certain pattern. Commonly used for phone numbers, zip codes, etc.

preg_match()
  • PHP function used to search a string.
  • Returns a 1 if a match is found.
  • If the pattern is not found a 0 will be returned.
Format
preg_match(/search_pattern/, $your_string);
// where search_pattern needs to be a regular expression and $your_string is the string to test.
Creating a search pattern.

Search patterns are delineated by slashes. /search_pattern/

Some or all of the following can be used to create a search pattern.

Regular Expression Examples
Regular Expression Flags
i
 Case Insensitive
g
 Global (all matches)
Regular Expression Character Positions
^
 beginning of the text string
$
 end of the text string
\b
 presence of a word boundary
\B
 absence of a word boundary
Regular Expression Character Classes
\d
 digit (0-9)
\D
 non-digit
\w
 word character (a-z, A-Z, 0-9, _)
\W
 non-word character (special characters)
\s
 white-space character (blank space, tab, newline, carriage return, or formfeed
\S
 non-white spce character
.
 any character
[chars]
 match any character in the list of characters chars
[^chars]
 match any character not in the list of characters chars
[char1-charN]
 match any character in the range char1 - charN
[^char1-charN]
 match any character not in the range char1 - charN
[a-z]
 match lowercase letters
[A-Z]
 match uppercase letters
[a-zA-Z]
 match letters (uppercase or lowercase)
[0-9]
 match digits
Repetition Characters
*
 Repeat 0 or more times
?
 Repeat 0 or 1 time
+
 Repeat 1 or more times
{n}
 Repeat exactly n times
{n,}
 Repeat at least n times
{n,m}
 Repeat at least n times but no more than m times
Escape Sequences
\/
 /
\\
 \
\.
 .
\*
 *
\+
 +
\?
 ?
\|
 |
\(\)
 ()
\{\}
 {}
\^
 ^
\$
 $
\n
 new line
\r
 carriage return
\t
 tab
Example
if (empty($_POST['email'])){
	// check for missing email
	$emailError = '<span class="text-danger">Required</span>';
	$valid = false;
} else {
	$email = trim($_POST['email']);
	// validate email using a regular expression
	if (!preg_match('/[-\w.]+@([A-z0-9][-A-z0-9]+\.)+[A-z]{2,4}/', $email)) {
		// returns 1 (true) for match, 0 (false) for no match
		$emailError = "<p class='error'>Invalid email address</p>";
		$valid = false;
	}
}

Above is an example for checking an email from a form with a regular expression to determine if it matches the pattern of a valid email address. This does not verify an actual working email, only that it matches the pattern (user@domain.tld) of a valid email address. You can try out more regular expressions with the regular expression tester.

Useful Regular Expressions
U.S. Zip Code \d{5}(-\d{4})?
U.S. Phone Number \(?(\d{3})\)?[ -.](\d{3})[ -.](\d{4})
Email address [-\w.]+@([A-z0-9][-A-z0-9]+\.)+[A-z]{2,4}
Date ([01]?\d)[-\/ .]([123]?\d)[-\/ .](\d{4})
Web address ((\bhttps?:\/\/)|(\bwww\.))\S*