Regex Word Boundary

Summary: in this tutorial, you’ll learn to use the regex word boundary to match the word boundary position in a string.

Introduction to the regex word boundary

The word boundary anchor \b matches a position called a word boundary in a string.

A word boundary’s position can be one of the following:

  • Before the first character if it’s a word character in a string.
  • After the last character if it’s a word character in a string.
  • Between two characters in a string if the first character is a word character and the other is not.

The following picture illustrates the word boundary positions in the string "PHP IS COOL!":

In other words, the word boundary allows you to match the whole word using the regular expression:

/\bword\b/
Code language: PHP (php)

The following example matches PHP in the string:

<?php $pattern = '/PHP/'; $title = 'PHP is awesome. How is CakePHP?'; if (preg_match_all($pattern, $title, $matches)) { print_r($matches[0]); }
Code language: PHP (php)

Output:

Array ( [0] => PHP [1] => PHP )
Code language: PHP (php)

It returns the first match as PHP and the second as PHP in the CakePHP. However, if you add the word boundary (\b) to the regular expression, it matches only the word PHP:

<?php $pattern = '/\bPHP\b/'; $title = 'PHP is awesome. How is CakePHP?'; if (preg_match_all($pattern, $title, $matches)) { print_r($matches[0]); }
Code language: PHP (php)

Output:

Array ( [0] => PHP )
Code language: PHP (php)

Summary

  • Use the regex word boundary anchor to match a word boundary in a string.
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