Recipe 22.8. Escaping Special Characters in a Regular Expression

22.8.1. Problem

You want to have characterssuch as * or + treated as literals, not as metacharacters, inside a regular expression. This is useful when allowing users to type in search strings you want to use inside a regular expression.

22.8.2. Solution

Use preg_quote( ) to escape Perl-compatible regular-expression metacharacters:

<?php $pattern = preg_quote('The Education of H*Y*M*A*N K*A*P*L*A*N').':(\d+)'; if (preg_match("/$pattern/",$book_rank,$matches)) {     print "Leo Rosten's book ranked: ".$matches[1]; } ?>

Use quotemeta( ) to escape POSIX metacharacters:

$pattern = quotemeta('M*A*S*H').':[0-9]+'; if (ereg($pattern,$tv_show_rank,$matches)) {     print 'Radar, Hot Lips, and the gang ranked: '.$matches[1]; }

22.8.3. Discussion

Here are the characters that preg_quote( ) escapes:

. \ + * ? ^ $ [ ] () { } < > = ! | :

Here are the characters that quotemeta( ) escapes:

. \ + * ? ^ $ [ ] ()

These functions escape the metacharacters with backslash.

The quotemeta( ) function doesn't match all POSIX metacharacters. The characters {, }, and | are also valid metacharacters but aren't converted. This is another good reason to use preg_match( ) instead of ereg( ).

You can also pass preg_quote( ) an additional character to escape as a second argument. It's useful to pass your pattern delimiter (usually /) as this argument so it also gets escaped. This is important if you incorporate user input into a regular expression pattern. The following code expects $_GET['search_term'] from a web form and searches for words beginning with $_GET['search_term'] in a string $s:

$search_term = preg_quote($_GET['search_term'],'/'); if (preg_match("/\b$search_term/i",$s)) {    print 'match!'; }

Using preg_quote( ) ensures the regular expression is interpreted properly if, for example, a Magnum, P.I. fan enters t.c as a search term. Without preg_quote( ), this matches tic, tucker, and any other words whose first letter is t and third letter is c. Passing the pattern delimiter to preg_quote( ) as well makes sure that user input with forward slashes in it, such as CP/M, is also handled correctly.

22.8.4. See Also

Documentation on preg_quote( ) at http://www.php.net/preg-quote and quotemeta( ) at http://www.php.net/quotemeta.