Regular expressions (regex) allow defining patterns to match in text strings using special characters like ., *, + to match types of characters or ranges. They have components like literal characters, character classes, shorthand classes, quantifiers and modifiers that define what is matched. PHP supports both PCRE and POSIX regex with functions like preg_match() and ereg_replace() as well as online tools for testing regular expressions.

1. Regular Expression
What are Regular Expressions?
Regular Expressions (regex) are a way to define a
pattern of characters in a text string.

2. Not all REGEX are equal
PCRE : Perl Compatible Regex
Posix : (Simple Regular Expression)
Basic Regex (BRE)
Extended Regex (ereg_...) (1986!)
deprecated as of PHP 5.3.0

3. Components
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Literal Characters
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Character Classes
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Shorthand Character Classes / 'dot'
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Non-Printable Characters / Anchors
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Quantifiers
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Modifiers

4. Literal Characters
Are simple characters the way you expect
them.
e.g. /a/ -> will match 'a' in a text

5. Character Classes
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Groups of character
[abcdefgh0123]
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Range of character
[a-h0-3]
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Inverse Range of Character
[^i-z4-9]
e.g. /[gG]uide[lL]ine/ -> guideline or
guideLine

6. Shorthand Char. Classes
d -> [0-9]
w -> [a-zA-Z]
s -> whitespace + [trn]
negative:
D -> [^0-9]
W -> [^a-zA-Z]
S
'.' -> any character!!

7. Non-printable Characters
t -> tab character (ASCII 0x09)
r -> carriage return (0x0D)
n -> line feed (0x0A)
(a (bell, 0x07), e (escape, 0x1B), f (form feed, 0x0C) ,v (vertical
tab, 0x0B) )
xFF -> hexadecimal index in the char. set
e.g. xA9 -> copyright symbol in the Latin-1
uFFFF -> Unicode character
e.g. u20AC -> the euro currency sign
^ begin of the string
$ end of the string
b -> word boundary - B -> not word boundary

8. Quantifiers
REGEXES ARE GREEDY
{min, max} / {min,} / {,max} / {exact}
? -> {0,1}
+ -> {1,}
* -> {0,}
lazy quantifiers:
carefull when using /.*/
+?
*?
| : not quantifier, simple 'OR'

9. Modifiers
//i : case incensitive
//m : multiline
//x : ignore whitespace
Internal Option Set:
(? .. )
(?i)
e.g. /ab(?i)c/ -> "abc" and "abC"

10. Subpatterns
Pattern in a pattern in .....
Can be nested!!
e.g. /((red|white) (king|queen))/
reg king
white king
red queen
white queen

11. PHP & Regex
preg_.... : PCRE
strpos() or strstr() faster
ereg_.... : Posix
deprecated in 5.3.0
preg_ is often faster
mb_ereg_...: "multibyte"

12. Resources & Tools
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http://www.regular-expressions.info
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http://en.wikipedia.org/wiki/Regular_expression
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http://be.php.net/manual/en/regexp.reference.php
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http://regexpal.com/
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http://www.fileformat.info/tool/regex.htm
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http://www.regexbuddy.com/
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http://www.ultrapico.com/Expresso.htm