Basic regexes with preg_match() and preg_match_all()

int preg_match ( string pattern, string subject [, array matches [, int flags [, int offset]]])

int preg_match_all ( string pattern, string subject [, array matches [, int flags [, int offset]]])

Our basic regex function is preg_match() and takes two parameters: the pattern to match, and the string to match it against. Preg_match() will apply the regular expression in parameter one to the string in parameter two and see whether it finds a match - if it does, it will return 1, otherwise 0. The reason it returns 1 is because regular expressions return the number of matches found, but preg_match(), for speed reasons, returns as soon as it finds the first match - this means it is very quick to check whether a pattern exists in a string. An alternative function, preg_match_all(), does not exit after the first match - we will get onto that later.

Regular expressions are formed by starting with a forward slash /, followed by a sequence of special symbols and words to match, then another slash and, optionally, a string of letters that affect the expression.

Sound complicated? Don't worry - it is complicated! We're going to break it right down, though, so hopefully it will make sense immediately. Here is a list of very basic regular expressions, strings, and whether or not a match is made:

Regex	String	Result
/php/	php	Match
php/	php	Error; you need a slash at the start
/php/	PHP	No match; regexes are case sensitive
/php/i	PHP	Match; /i means "case insensitive"
/Foo/i	FOO	Match

As you can see, "i" is the modifier to make the regex case insensitive - it will slow things down a touch, but might make your life a great deal easier.

Here is how to use preg_match() on some of those example regexes:

<?php
    if (preg_match("/php/", "php")) {
        print "Got match!\n";
    }

    if (preg_match("/php/", "PHP")) {
        print "Got match!\n";
    }

    if (preg_match("/php/i", "php")) {
        print "Got match!\n";
    }
?>

The first and third preg_match() calls will return true (the second will fail because the case is wrong and "i" is not specified), and print "Got match!"

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