preg_quote() - Escape Characters in Pattern

PHP Modules Tutorials - Herong's Tutorial Examples

∟PCRE Module - Perl Compatible Regular Expressions

∟preg_quote() - Escape Characters in Pattern

This section provides a tutorial example on how to quote special characters in a string to be used as a regular expression pattern using the preg_quote() function.

Regular expression pattern uses a set of reserved characters, ". \ + * ? [ ^ ] $ ( ) { } = ! < > | : - #", to provide special functionalities. If you want to match any one of them in the subject string, you need to escape it using a backslash. This is called quoting a special character. For example, "\." is a quoted special character that matches a single ".". "\\" is a quoted special character that matches a single "\".

If you want to use the string stored in a variable as the search pattern, you should call the preg_quote() function to quote any special characters in the string:

$quoted = preg_quote($pattern, $delimiter);

where: 
  $pattern: String to be quoted
  $delimiter: Delimiter character to be quoted as special character
  $quoted: Quoted string

Here is an example script that shows the difference between un-quoted and quoted strings:

<?php
# preg-quote-escape-character.php
# Copyright 2009-2024 (c) HerongYang.com. All Rights Reserved.

  $url = "https://herongyang.com/PHP/index.html";
  $path = "/PHP/index.html";

  print("URL: $url\n");
  print("Path: $path\n");

  print("Remove path as unquoted pattern:\n");
  $removed = preg_replace("/$path/", "", $url);	
  print("  URL without path: $removed\n");

  print("Remove path as quoted pattern:\n");
  $quoted = preg_quote($path, "/");
  $removed = preg_replace("/$quoted/", "", $url);	
  print("  Quoted path: $quoted\n");
  print("  URL without path: $removed\n");
?>

If you run the above script, you will see the following output:

herong$ php preg-quote-escape-character.php 

URL: https://herongyang.com/PHP/index.html
Path: /PHP/index.html

Remove path as unquoted pattern:
PHP Warning:  preg_replace(): Unknown modifier 'P' in 
  /Users/herong/preg-quote-escape-character.php on line 12
  URL without path: 

Remove path as quoted pattern:
  Quoted path: \/PHP\/index\.html
  URL without path: https://herongyang.com

As you can see from the output, using the un-quoted string as the search pattern failed in the preg_match() call, because the first un-quoted character '/' inside the pattern string is considered as the ending delimiter. The "P" character after "/" is considered as a modifier and is invalid.