13.9. Escaping Special Characters in a Regular Expression13.9.1. ProblemYou want to have characters such 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. 13.9.2. SolutionUse preg_quote( ) to escape Perl-compatible regular-expression metacharacters: $pattern = preg_quote('The Education of H*Y*M*A*N K*A*P*L*A*N').':(\d+)'; if (preg_match("/$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]; } 13.9.3. DiscussionHere 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 $_REQUEST['search_term'] from a web form and searches for words beginning with $_REQUEST['search_term'] in a string $s: $search_term = preg_quote($_REQUEST['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. 13.9.4. See AlsoDocumentation on preg_quote( ) at http://www.php.net/preg-quote and quotemeta( ) at http://www.php.net/quotemeta. Copyright © 2003 O'Reilly & Associates. All rights reserved. |
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