PHP Regular Expressions Finding the nth Occurrence of a Match

PHP Regular Expressions

Finding the nth Occurrence of a Match

Problem

You want to find the nth word match instead of the first one.

Solution

Example  Finding the nth match

       $todo = "1. Get Dressed 2. Eat Jelly 3. Squash every week into a day";

       preg_match_all("/\d\. ([^\d]+)/", $todo, $matches);

       print "The second item on the todo list is: ";

       // $matches[1] is an array of each substring captured by ([^\d]+)

       print $matches[1][1] . "\n";

       print "The entire todo list is: ";

       foreach($matches[1] as $match) {

            print "$match\n";

       }

Discussion

Because the preg_match() function stops after it finds one match, you need to use preg_match_all() instead if you’re looking for additional matches. The preg_match_all() function returns the number of full pattern matches it finds. If it finds no matches, it returns 0. If it encounters an error, such as a syntax problem in the pattern, it returns false.

The third argument to preg_match_all() is populated with an array holding information about the various substrings that the pattern has matched. The first element holds an array of matches of the complete pattern. For Example this means that $match es[0] holds the parts of $todo that match /\d\. ([^\d]+)/: 1. Get Dressed, 2. Eat Jelly, and 3. Squash every week into a day.

Subsequent elements of the $matches array hold arrays of text matched by each parenthesized subpattern. The pattern in example has just one subpattern ([^\d\]+). So $matches[1] is an array of strings that match that subpattern: Get Dressed, Eat Jelly, and Squash every week into a day.

If there were a second subpattern, the substrings that it matched would be in $matches[2], a third subpattern’s matches would be in $matches[3], and so on.

Instead of returning an array divided into full matches and then submatches, preg_match_all() can return an array divided by matches, with each submatch inside. To trigger this, pass PREG_SET_ORDER in as the fourth argument. This is particularly useful when you’ve got multiple captured subpatterns and you want to iterate through the subpattern groups one group at a time.

Example  Grouping captured subpatterns

       $todo = "

       first=Get Dressed

       next=Eat Jelly

       last=Squash every week into a day

       ";

       preg_match_all("/([a-zA-Z]+)=(.*)/", $todo, $matches, PREG_SET_ORDER);

       foreach ($matches as $match) {

            print "The {$match[1]} action is {$match[2]}\n";

       }

Example  prints:

       The first action is Get Dressed

       The next action is Eat Jelly

       The last action is Squash every week into a day

With PREG_SET_ORDER, each value of $match in the foreach loop contains all the subpatterns: $match[0] is the entire matched string, $match[1] the bit before the =, and $match[2] the bit after the =.