Functions of Strings and Regular Expressions

PHP’s Regular Expression Functions (Perl Compatible)

PHP offers seven functions for searching strings using Perl-compatible regular expressions:preg_grep(),preg_match(),preg_match_all(),preg_quote(),preg_replace(),preg_replace_callback(), andpreg_split(). These functions are introduced in the following sections.

Searching an Array

Thepreg_grep()function searches all elements of an array, returning an array consisting of all elements matching a certain pattern. Its prototype follows:

array preg_grep(string pattern, array input [, flags])

Consider an example that uses this function to search an array for foods beginning withp:

<?php
    $foods = array("pasta", "steak", "fish", "potatoes");
    $food = preg_grep("/^p/", $foods);
    print_r($food);
?>

This returns the following:

--------------------------------------------
Array ( [0] => pasta [3] => potatoes )
--------------------------------------------

Note that the array corresponds to the indexed order of the input array. If the value at that index position matches, it’s included in the corresponding position of the output array. Otherwise, that position is empty. If you want to remove those instances of the array that are blank, filter the output array through the functionarray_values(), introduced in Chapter 5.

The optional input parameterflagswas added in PHP version 4.3. It accepts one value,PREG_GREP_INVERT. Passing this flag will result in retrieval of those array elements that do not match the pattern.

Searching for a Pattern

Thepreg_match()function searches a string for a specific pattern, returningTRUEif it exists, andFALSEotherwise. Its prototype follows:

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

The optional input parameterpattern_arraycan contain various sections of the subpatterns contained in the search pattern, if applicable. Here’s an example that usespreg_match()to perform a case-insensitive search:

<?php
    $line = "vim is the greatest word processor ever created!";
    if (preg_match("/\bVim\b/i", $line, $match)) print "Match found!";
?>

For instance, this script will confirm a match if the wordVimorvim is located, but notsimplevim,vims, orevim.

Matching All Occurrences of a Pattern

Thepreg_match_all()function matches all occurrences of a pattern in a string, assigning each occurrence to an array in the order you specify via an optional input parameter. Its prototype follows:

int preg_match_all(string pattern, string string, array pattern_array
                   [, int order])

Theorder parameter accepts two values:

1. PREG_PATTERN_ORDERis the default if the optionalorder parameter is not included.PREG_PATTERN_ORDERspecifies the order in the way that you might think most logical:$pattern_array[0]is an array of all complete pattern matches,$pattern_array[1]is an array of all strings matching the first parenthesized regular expression, and so on.
   
2. PREG_SET_ORDERorders the array a bit differently than the default setting.$pattern_array[0]contains elements matched by the first parenthesized regular expression,$pattern_array[1]contains elements matched by the second parenthesized regular expression, and so on.

Here’s how you would usepreg_match_all()to find all strings enclosed in bold HTML tags:

<?php
    $userinfo = "Name: <b>Zeev Suraski</b> <br> Title: <b>PHP Guru</b>";
    preg_match_all("/<b>(.*)<\/b>/U", $userinfo, $pat_array);
    printf("%s <br /> %s", $pat_array[0][0], $pat_array[0][1]);
?>

This returns the following:

--------------------------------------------
Zeev Suraski
PHP Guru
--------------------------------------------