Function to escape regex patterns before applied in PHP

Last Updated : 31 Oct, 2019

Use preg_quote() function in PHP to escape regex patterns before it is applied in run time. The preg_quote() function puts a backslash in front of every character within the specified string that would be a part of the regex syntax in PHP, thereby making them escape sequences. These characters include:
. \ + * ? [ ^ ] $ ( ) { } = ! | : – #

Syntax:

string preg_quote( string $str, string $delimiter = NULL )

Parameters:

$str: This parameter holds the input string.
$delimiter: It is optional parameter. The most common delimiter is ‘/’ as it is not a special regex character that would be normally handled by preg_quote(). Primarily used in conjunction with preg_replace() function.

Returns: It returns the delimiter containing string.

Below programs illustrates the preg_quote() function.

Program 1:

<?php 
  
// Create a string which need to be escaped 
$str = "Welcome to GfG! (+ The course fee. $400) /"; 
      
echo "Before Processing - " . $str . PHP_EOL; 
  
// Use preg_quote() on above string 
$processedStr = preg_quote($str); 
  
// Display the output 
echo "After Processing - " . $processedStr; 
  
?> 

Output:

Before Processing - Welcome to GfG! (+ The course fee. $400) /
After Processing - Welcome to GfG\! \(\+ The course fee\. \$400\) /

Notice that a backslash was put in front of every special character, but not for the forward slash. For that, we can use the delimiter, see the program below:

Program 2

<?php 
  
// Create a string which need to be escaped 
$str = "Welcome to GfG! (+ The course fee. $400) /"; 
      
echo "Before Processing - " . $str . PHP_EOL; 
  
// Use preg_quote() on above string 
$processedString = preg_quote($str, '/'); 
  
// Display the output 
echo "After Processing - " . $processedString; 
  
?> 

Output:

Before Processing - Welcome to GfG! (+ The course fee. $400) /
After Processing - Welcome to GfG\! \(\+ The course fee\. \$400\) \/

Below program demonstrates the combined use of preg_quote() and preg_replace() function.

Program 3:

<?php 
  
// PHP Program emphasize the word within * * and  
// use font style to italic within [ ] using 
// both preg_quote() and preg_replace() function 
  
$str = "The *article* was written by [GFG]"; 
      
// The words to be formatted 
$word1 = "*article*"; 
$word2 = "[GFG]"; 
      
// The first string only applies bold to word 1, 
// preg_quote() escapes the * * 
$processedStr1 = preg_replace("/" . preg_quote($word1, '/') 
            . "/", "<strong>" . $word1 . "</strong>", $str); 
  
echo "BOLD ONLY - " . $processedStr1 . PHP_EOL; 
  
// The second string only applies italics to  
// word 2, preg_quote() escapes the [ ] 
$processedStr2 = preg_replace("/" . preg_quote($word2, '/') 
            . "/", "<em>" . $word2 . "</em>", $str); 
  
echo "ITALIC ONLY - " . $processedStr2 . PHP_EOL; 
  
// Combining both text formattings and display 
$bothReplacementsCombined = preg_replace("/" . 
            preg_quote($word2, '/') . "/", 
            "<em>" . $word2 . "</em>", $processedStr1); 
  
echo "BOTH COMBINED - " . $bothReplacementsCombined; 
  
?> 

Output:

BOLD ONLY - The *article* was written by [GFG]
ITALIC ONLY - The *article* was written by [GFG]
BOTH COMBINED - The *article* was written by [GFG]

Note: To notice the application of the text formatting tags, you should run PHP on your local server and echo to the browser.

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