PHP: Escape RegEx pattern to prevent being applied?

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preg_quote() is what you are looking for:

Description

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

preg_quote() takes str and puts a
backslash in front of every character
that is part of the regular expression
syntax. This is useful if you have a
run-time string that you need to match
in some text and the string may
contain special regex characters.

The special regular expression
characters are: . \ + * ? [ ^ ] $ ( ) { } = ! < > | : -

Parameters

str

The input string.

delimiter

If the optional delimiter is specified, it will also be escaped. This is useful for escaping the delimiter that is required by the PCRE functions. The / is the most commonly used delimiter.

Importantly, note that if the $delimiter argument is not specified, the delimiter – the character used to enclose your regex, commonly a forward slash (/) – will not be escaped. You will usually want to pass whatever delimiter you are using with your regex as the $delimiter argument.

Example – using preg_match to find occurrences of a given URL surrounded by whitespace:

$url="http://stackoverflow.com/questions?sort=newest";

// preg_quote escapes the dot, question mark and equals sign in the URL (by
// default) as well as all the forward slashes (because we pass "https://stackoverflow.com/" as the
// $delimiter argument).
$escapedUrl = preg_quote($url, "https://stackoverflow.com/");

// We enclose our regex in "https://stackoverflow.com/" characters here - the same delimiter we passed
// to preg_quote
$regex = '/\s' . $escapedUrl . '\s/';
// $regex is now:  /\shttp\:\/\/stackoverflow\.com\/questions\?sort\=newest\s/

$haystack = "Bla bla http://stackoverflow.com/questions?sort=newest bla bla";
preg_match($regex, $haystack, $matches);

var_dump($matches);
// array(1) {
//   [0]=>
//   string(48) " http://stackoverflow.com/questions?sort=newest "
// }

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