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# preg_match

(PHP 3>= 3.0.9, PHP 4 , PHP 5)

preg_match -- Perform a regular expression match

## Description

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

Searches subject for a match to the regular expression given in pattern.

If matches is provided, then it is filled with the results of search. $matches[0] will contain the text that matched the full pattern, $matches[1] will have the text that matched the first captured parenthesized subpattern, and so on.

flags can be the following flag:

PREG_OFFSET_CAPTURE

  If this flag is passed, for every occurring match the appendant string offset will also be returned. Note that this changes the return value in an array where every element is an array consisting of the matched string at offset 0 and its string offset into subject at offset 1. This flag is available since PHP 4.3.0 .

The flags parameter is available since PHP 4.3.0.

Normally, the search starts from the beginning of the subject string. The optional parameter offset can be used to specify the alternate place from which to start the search. It is equivalent to passing substr()($subject, $offset) to preg_match() in place of the subject string. The offset parameter is available since PHP 4.3.3.

preg_match() returns the number of times pattern matches. That will be either 0 times (no match) or 1 time because preg_match() will stop searching after the first match. preg_match_all() on the contrary will continue until it reaches the end of subject. preg_match() returns `FALSE` if an error occurred.

> 心得: Do not use preg_match() if you only want to check if one string is contained in another string. Use strpos() or strstr() instead as they will be faster.

示範 1.Find the string of text "php"
`<?php // The "i" after the pattern delimiter indicates a case-insensitive search if (preg_match("/php/i", "PHP is the web scripting language of choice.")) { echo "A match was found."; } else { echo "A match was not found."; } ?>`

示範 2.Find the word "web"
`<?php /* The \b in the pattern indicates a word boundary, so only the distinct * word "web" is matched, and not a word partial like "webbing" or "cobweb" */ if (preg_match("/\bweb\b/i", "PHP is the web scripting language of choice.")) { echo "A match was found."; } else { echo "A match was not found."; } if (preg_match("/\bweb\b/i", "PHP is the website scripting language of choice.")) { echo "A match was found."; } else { echo "A match was not found."; } ?>`

示範 3.Getting the domain name out of a URL
`<?php // get host name from URL preg_match("/^(http:\/\/)?([^\/]+)/i", "http://www.php.net/index.html", $matches); $host = $matches[2]; // get last two segments of host name preg_match("/[^\.\/]+\.[^\.\/]+$/", $host, $matches); echo "domain name is: {$matches[0]}\n"; ?>`
This example will produce:
    domain name is: php.net

See also preg_match_all(), preg_replace(), and preg_split().

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