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Common used regular expressions

Regular expressions are cumbersome but powerful. Understanding regular expressions can improve efficiency and also give you the absolute sense of accomplishment.

Here is a brief reference sheet for common used regular expressions:

Character Definition Example
^ The pattern has to appear at the beginning of a string. ^cat matches any string that begins with cat
$ The pattern has to appear at the end of a string. cat$ matches any string that ends with cat
. Matches any character. cat. matches catT and cat2 but not catty
[ ] Bracket expression. Matches one of any characters enclosed. gr[ae]y matches gray or grey
[^] Negates a bracket expression. Matches one of any characters EXCEPT those enclosed. 1[^02] matches 13 but not 10 or 12
[-] Range. Matches any characters within the range. [1-9] matches any single digit EXCEPT 0
? Preceeding item must match one or zero times. colou?r matches color or colour but not colouur
+ Preceeding item must match one or more times. be+ matches be or bee but not b
* Preceeding item must match zero or more times. be* matches b or be or beeeeeeeeee
{} Parentheses. Creates a substring or item that metacharacters can be applied to a(bee)?t matches at or abeet but not abet
{n} Bound. Specifies exact number of times for the preceeding item to match. [0-9]{3} matches any three digits
{n,} Bound. Specifies minimum number of times for the preceeding item to match. [0-9]{3,} matches any three or more digits
{n,m} Bound. Specifies minimum and maximum number of times for the preceeding item to match. [0-9]{3,5} matches any three, four, or five digits
| Alternation. One of the alternatives has to match. July (first|1st|1) will match July 1st but not July 2

Perl-Style Metacharacters

Character Definition Example
// Default delimiters for pattern /colou?r/ matches color or colour
i Append to pattern to specify a case insensitive match /colou?r/i matches COLOR or Colour
\b A word boundary, the spot between word (\w) and non-word (\W) characters /\bfred\b/i matches Fred but not Alfred or Frederick
\B A non-word boundary /fred\B/i matches Frederick but not Fred
\d A single digit character a\db/i matches a2b but not acb
\D A single non-digit character /a\Db/i matches aCb but not a2b
\n The newline character. (ASCII 10) /\n/ matches a newline
\r The carriage return character. (ASCII 13) /\r/ matches a carriage return
\s A single whitespace character /a\sb/ matches a b but not ab
\S A single non-whitespace character /a\Sb/ matches a2b but not a b
\t The tab character. (ASCII 9) /\t/ matches a tab.
\w A single word character - alphanumeric and underscore /\w/ matches 1 or _ but not ?
\W A single non-word character /a\Wb/i matches a!b but not a2b

The followings are some examples of usig regular expressions

Date and Time

Time format (no seconds): HH:MM am/pm


Date in mm/dd/yyyy format, with an option for m/d/yyyy (exclude zero’s)

^(0?[1-9]|1[012])[ \/.-](0?[1-9]|[12][0-9]|3[01])[ \/.-](19|20)\d\d$

Date in dd/mm/yyyy format, with an option for d/m/yyyy (exclude zero’s)

^(0?[1-9]|[12][0-9]|3[01])[ \/.-](0?[1-9]|1[012])[ \/.-](19|20)\d\d$

Phone number

Parenthesis, periods, dashes, underscore, and spaces are allowed in phone number: (123)456-7890
(123) 456 – 7890
( 123 )456-7890
123 456 7890


Postal code

Matches Canadian postal code formats with or without spaces (e.g., "T2X 1V4" or "T2X1V4")

^[ABCEGHJKLMNPRSTVXY]{1}\d{1}[A-Z]{1} *\d{1}[A-Z]{1}\d{1}$	

Validate domain name


Validate email format

It verifies if the email is written in the following


Checking password complexity

This regular expression will tests if the input consists of 6 or more letters, digits, underscores and hyphens. The input must contain at least one upper case letter, one lower case letter and one digit.


Highlight a word from a text

$text = "Sample text here";
$text = preg_replace("/b(regex)b/i", '<span style="background:#f00">1</span>', $text);
echo $text;

Remove repeated words (case insensitive)

'Keep your your head' becomes 'Keep your head'

$text = preg_replace("/\s(\w+\s)\1/i", "$1", $text);

Remove repeated punctuation

'Keep your head......' becomes 'Keep your head.'

$text = preg_replace("/\.+/i", ".", $text); 

Get all image urls from an html document

Puts all the image URLs in an array.

$images = array();
preg_match_all('/(img|src)\=(\"|\')[^\"\'\>]+/i', $data, $media);
foreach($data as $url)
	$info = pathinfo($url);
	if (isset($info['extension']))
		if (($info['extension'] == 'jpg') || 
		($info['extension'] == 'jpeg') || 
		($info['extension'] == 'gif') || 
		($info['extension'] == 'png'))
		array_push($images, $url);