PHP-Regular Expressions

Think of regular expressions as an elaborate system of matching patterns. You first write the pattern, then use one of PHP's built-in functions to apply the pattern to a text string regular expressions are specifically for use with strings. PHP has essentially two functions for using regular expressions to match patterns one case sensitive and one not and two for matching patterns and replacing matched text with other text again, one case sensitive and one not

Some text editors, such as BBEdit for Macintosh, TextPad for Windows and emacs for Unix, allow you to use regular expressions to match and replace patterns within and throughout several documents.

This may be another good reason to learn regular expressions and is perhaps something to consider when choosing your text editor.

This is a fairly complete list of special characters used to define your regular expression patterns (including metacharacters but not literals—a, b, c, etc.).

Special Characters for Regular Expression
Character	Matches
.	any character
^a	begins with a
a$	ends with a
a+	at least one a
a?	zero or one a
\n	new line
\t	tab
\	escape
(ab)	ab grouped
a|b	a or b
a{ 2}	aa
a{ 1,}	a, aa, aaa, etc.
a{ 1,3}	a, aa, aaa
[a-z]	any lowercase letter
[A-Z]	any uppercase letter
[0-9]	any digit

Regular expressions also make use of the pipe | as the equivalent of or. Therefore, "a|b" will match the strings a or b and "gre|ay" matches both potential spellings of the color. Using the pipe within patterns is called alternation.

Practically, of course, there's little use to matching repetitions of a letter in a string, but these examples are good ways to demonstrate how a symbol works. You should begin by focusing on understanding what the various symbols mean and how they are used. 

To include special characters (^.[]$()|*?{ } \) in a pattern, they need to be escaped (a backslash put before them). This is true for the metacharacters and the grouping symbols (parenthesis and brackets). You can also use the backslash to match new lines ("\n") and tabs ("\t"), essentially creating a metacharacter out of a literal.

There are two functions built in to PHP expressly for the purpose of matching a pattern within a string: ereg() and eregi(). The only difference between the two is that ereg() treats patterns as case-sensitive whereas eregi() is case-insensitive, making it less particular. The latter is generally recommend for common use, unless you need to be more explicit (perhaps for security purposes, as with passwords). Both functions will be evaluated to TRUE if the pattern is matched, FALSE if it is not. Here are two different ways to use these functions:

ereg("pattern", "string");

Or:

$Pattern = "pattern";
$String = "string";
eregi($Pattern, $String);

Throughout the rest of the chapter, I will assign the pattern to a variable, as in the second example above, to draw more attention to the pattern itself—the heart of any regular expression.