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PHP Programming
By James N Hitz
How to create Regular Expressions
A regular expression is made up of several special characters. These work more or less like MS-DOS; * and? (but hereby with different meanings).
These special characters are placeholders and represent other characters. The most useful of these include:
. - any character
x? - zero or one occurence of character "x"
x* - zero or more occurences of character "x"
x+ - one or more occurences of character "x"
\n - new line
\r - carriage return
\s - space character (\t \n \r or \f)
\t - tab
\d - digit save as [0-9]
\D - non-digit save as [^0-9]
\w - word character [A-Za-Z0-9_]
\W - non-word
a|b|c - letter 'a' or 'b' or 'c'
x{v} - v occurences of x
x{v,z} - v or z occurences of x
Anchors
Anchors in a regular expression define where within a string an expression should occur. The most common PHP anchors include:
^ - begning of string $ - and of a string \b - word boudary
Where one character from a 'pool' of more than one characters is to be matched, the 'pool' is created by enclosing the particular characters in squre brackets eg. [aeiou] to test for ANY one vowel or [aeiou]+ to test for ONE or MORE occurences of any vowels.
A caret(^) may be added to denounce any of the character enclosed in square brackets eg [^aeiou] to return true for any one NON-VOWEL (Consanant) character.
Please note that the usage of a caret (^) can be confusing because as you may have noted, it has 2 uses:
- to test inequality when enclosed within square brackets eg. [^a-z]
- to test for the beginning of a string when used outside of square brackets eg. /^s/ ie. as an anchor
Parenthesis () when used in a regular expression have a special meaning in that they create special variables that can later be referenced. Let's take an example:
... $string ="chmod 711"; $string =~/(/w+)\s+(\d+)/; ...
The (\w+) tests for one more word characters and stores in a special PHP variables called $1. The expression further proceeds to test for any number of spaces using \s+. At the end of the string, the regular expression tests for any number of digits using (\d+) the results of which are stored in another special PHP variable - $2. Thus far, $1 = "chmod" and $2 = "711"
When braces ({ }/curly brackets) are used in a regular expresion, they may be used to test an occurence of a pattern existing a given number of times:
[a-z]{2} - indicates that there must only be 2 lowercase
alphabetic digits.
[a-z]{2,} - indicates that must be at least 2 lowercase
alphabetic digits.
[a-z]{2,4} - indicates that there must be EITHER two,
three or four lowercase alphabetics.
Let's look at examples of how PHP's in-built functions for manipulating regular expressions along with how they can be used in PHP.
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