Regular Expressions allow you to check if a string of text matches a particular pattern - for example to see if the data that a user has entered into a form looks like a Postcode / zipcode ... and then to extract the vital parts of the string that you're checking into separate variables.
Down the left here, I've written the regular expression to match a British Postcode - this involves describing what a postcode looks like in great detail.
Here are some of the elements:
- Starts with
- Capital Letter
- one or more of the previous item
- previous item is optional
- exactly 2 of the previous item
- ends with
The round brackets are known as "capture parentheses" - or in easy English, they're used to indicate which parts of the pattern match against interesting bits of the incoming string that you would like to capture into separate variables when you match.
There are complete books on regular expressions, but this summary to the right shows you the basic types of elements ...
or "zero width assersions" indicate thinsg like "starts with" and "ends with" so you can match at the beginning, in the middle, at the end of, or a whole string.
match a single character (perhaps modified by a following count) exactly.
match any one character from a whole group of possibilities (perhaps modified by a following count) exactly.
say how many times the previous element may occur - the default is once.
You can also use (...) to group elements for capture, | which means "or" (a.k.a. alternation) and a few other types.
There are two regular expression engines in PHP - there's the ereg
series which are based on POSIX regular expressions, and the preg
series which are based on the Perl standard. In easy terms, the preg
flavour are shorter to write and more efficient in operation, but harder to understand and maintain.
One of the big questions with Regular Expressions is "are they case sensitive?" By default, yes they are, but you can add an "i" to ignore case - in different places in the syntax depending on if you're using ereg or preg.
If you're matching and capturing, you need to be very careful just which parts of the regular expression engine match which parts of the incoming string, as there can be many alternatives. You should remember that matching always starts as far to the left as it can, and that counts such as "+" always match as MANY characters as possible. If you use counts such as "+?" instead, then you're doing what is called a "sparse" rather than a "greedy" match, and that will match as few characters as possible.
The diagram shows the effect of sparse v greedy on a match to an email address with two @ characters in it. Most of the time, it'll be greedy you want to use ... sparse is useful especially when you're identifying / stripping HTML tags, though.
The boards / notes above come from the PHP Techniques Workshop
that I was presenting yesterday ... it's a regular public course and we would welcome YOU on it ;-) (written 2008-09-16)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articlesQ802 - Object Orientation and General technical topics - Regular Expression Elements 
Commenting Perl regular expressions - (2005-09-30) 
Next course - 7th January 2008, Regular Expressions - (2007-12-21) 
Diagrams to show you how - Tomcat, Java, PHP - (2008-08-22) 
String matching in Perl with Regular Expressions - (2008-10-20) 
Regular Expression Myths - (2010-06-13) 
Regular Expressions for the petrified - in Ruby - (2015-06-03) 
Regex Reference sheet - (2017-10-10)H107 - String Handling in PHP 
Here documents - (2004-08-28) 
PHP and natural sorting - (2004-09-19) 
the array returned by preg_match_all - (2005-06-06) 
PHP Magic Quotes - (2005-08-22) 
Splitting the difference - (2005-10-13) 
Running a Perl script within a PHP page - (2005-11-12) 
Converting between acres and hectares - (2006-01-08) 
The fencepost problem - (2006-01-10) 
PHP - dividing a string up into pieces - (2006-01-23) 
Robust PHP user inputs - (2006-02-03) 
Don't expose your regular expressions - (2006-02-15) 
How similar are two words - (2006-03-11) 
Evaluating arithmetic expressions in configuration files - (2006-05-10) 
Looking ahead and behind in a Regular Expression - (2006-05-22) 
Date conversion - PHP - (2006-12-26) 
PHP Regular expression to extrtact link and text - (2007-01-31) 
Regular Express Primer - (2007-05-20) 
Ignore case in Regular Expression - (2007-09-08) 
A taster PHP expression ... - (2007-09-30) 
Short and sweet and sticky - PHP form input - (2008-02-06) 
Do not SHOUT and do not whisper - (2008-04-06) 
Regular expression for 6 digits OR 25 digits - (2008-04-16) 
Finding variations on a surname - (2009-02-17) 
Making Regular Expressions easy to read and maintain - (2009-05-10) 
Handling nasty characters - Perl, PHP, Python, Tcl, Lua - (2009-06-14) 
Curly braces within double quoted strings in PHP - (2010-02-09) 
Handling (expanding) tabs in PHP - (2010-10-29) 
Divide 10000 by 17. Do you get 588.235294117647, 588.24 or 588? - Ruby and PHP - (2011-09-08) 
PHP - moving from ereg to preg for regular expressions - (2011-11-11) 
Regular Expression modifiers in PHP - summary table - (2011-11-12) 
Learning to program in PHP - Regular Expression and Associative Array examples - (2011-12-01) 
Getting more than a yes / no answer from a regular expression pattern match - (2012-06-30) 
More than just matching with a regular expression in PHP - (2012-06-30) 
Solution looking for a problem? Lookahead and Lookbehind - (2012-06-30) 
Setting up strings in PHP - (2013-04-27) 
Splitting the difference with PHP - (2013-04-27)
Some other Articles
FTP passive mode - a sometimes cure for upload hangsMotorcycles welcome at Well House ManorWill your backups work if you have to restore them?Spiders WebRegular Expressions in PHPWhat does an browser understand? What does an HTML document contain?I have been working hard but I do not expect you noticedlibwww-perl and Indy Library in your server logs?What have iTime, honeytrapagency and domain listing center got in common?Refactoring - a PHP demo becomes a production page