// PHP 5.4.1

// Either three or four \ can be used to match a '\'.
echo preg_match( '/\\\/', '\\' );        // 1
echo preg_match( '/\\\\/', '\\' );       // 1

// Match two backslashes `\\`.
echo preg_match( '/\\\\\\/', '\\\\' );   // Warning: No ending delimiter '/' found
echo preg_match( '/\\\\\\\/', '\\\\' );  // 1
echo preg_match( '/\\\\\\\\/', '\\\\' ); // 1

// Match one backslash using a character class.
echo preg_match( '/[\\]/', '\\' );       // 0
echo preg_match( '/[\\\]/', '\\' );      // 1  
echo preg_match( '/[\\\\]/', '\\' );     // 1

When using three backslashes to match a '\' the pattern below is interpreted as match a '\' followed by an 's'.

echo preg_match( '/\\\\s/', '\\ ' );    // 0  
echo preg_match( '/\\\\s/', '\\s' );    // 1  

When using four backslashes to match a '\' the pattern below is interpreted as match a '\' followed by a space character.

echo preg_match( '/\\\\\s/', '\\ ' );   // 1
echo preg_match( '/\\\\\s/', '\\s' );   // 0

The same applies if inside a character class.

echo preg_match( '/[\\\\s]/', ' ' );   // 0 
echo preg_match( '/[\\\\\s]/', ' ' );  // 1 

None of the above results are affected by enclosing the strings in double instead of single quotes.

Conclusions:
Whether inside or outside a bracketed character class, a literal backslash can be matched using just three backslashes '\\\' unless the next character in the pattern is also backslashed, in which case the literal backslash must be matched using four backslashes.

Recommendation:
Always use four backslashes '\\\\' in a regex pattern when seeking to match a backslash.

Escape sequences.

The backslash has a special meaning in both regexen and PHP. In both cases it is used as an escape character. For example, if you want to write a literal quote character inside a PHP string literal, this won't work:

$str = ''';

PHP would get "confused" which ' ends the string and which is part of the string. That's where \ comes in:

$str = '\'';

It escapes the special meaning of ', so instead of terminating the string literal, it is now just a normal character in the string. There are more escape sequences like \n as well.

This now means that \ is a special character with a special meaning. To escape this conundrum when you want to write a literal \, you'll have to escape literal backslashes as \\:

$str = '\\'; // string literal representing one backslash

This works the same in both PHP and regexen. If you want to write a literal backslash in a regex, you have to write /\\/. Now, since you're writing your regexen as PHP strings, you need to double escape them:

$regex = '/\\\\/';

One pair of \\ is first reduced to one \ by the PHP string escaping mechanism, so the actual regex is /\\/, which is a regex which means "one backslash".

There is a list of special Regex characters in the PHP documentation here: http://php.net/manual/en/function.preg-quote.php

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

If you have a string, you will need to escape the backslashes (and quotes) for the string literal. Or, depending on how the function builds the regex from the string, you might not need to escape slashes at all (I don't think so here).

"iframe=-*,src(match=\"/^(http:\\/\\/www\\.youtube\\.com\\/embed\\/)[^\\/\\s\\\\]+$/\")"

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 '/' as the
// $delimiter argument).
$escapedUrl = preg_quote($url, '/');

// We enclose our regex in '/' 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 "
// }