PHP Regex to get youtube video ID?

Use parse_url() and parse_str().

(You can use regexes for just about anything, but they are very easy to make an error in, so if there are PHP functions specifically for what you are trying to accomplish, use those.)

parse_url takes a string and cuts it up into an array that has a bunch of info. You can work with this array, or you can specify the one item you want as a second argument. In this case we're interested in the query, which is PHP_URL_QUERY.

Now we have the query, which is v=C4kxS1ksqtw&feature=relate, but we only want the part after v=. For this we turn to parse_str which basically works like GET on a string. It takes a string and creates the variables specified in the string. In this case $v and $feature is created. We're only interested in $v.

To be safe, you don't want to just store all the variables from the parse_url in your namespace (see mellowsoon's comment). Instead store the variables as elements of an array, so that you have control over what variables you are storing, and you cannot accidentally overwrite an existing variable.

Putting everything together, we have:

<?php
$url = "http://www.youtube.com/watch?v=C4kxS1ksqtw&feature=relate";
parse_str( parse_url( $url, PHP_URL_QUERY ), $my_array_of_vars );
echo $my_array_of_vars['v'];    
  // Output: C4kxS1ksqtw
?> 

Working example

Edit:

hehe - thanks Charles. That made me laugh, I've never seen the Zawinski quote before:

Some people, when confronted with a problem, think ‘I know, I’ll use regular expressions.’ Now they have two problems. – Jamie Zawinski

Using this pattern with a capturing group should give you the string you want:

d/(w+)?rel=d+"

example: https://regex101.com/r/kH5kA7/1

It seems that in the environment you have, the PCRE library was compiled without the PCRE_NEWLINE_ANY option, and $ in the multiline mode only matches before the LF symbol and . matches any symbol but LF.

You can fix it by using the PCRE (*ANYCRLF) verb:

'~(*ANYCRLF)S+(?=*$)~m'

(*ANYCRLF) specifies a newline convention: (*CR), (*LF) or (*CRLF) and is equivalent to PCRE_NEWLINE_ANY option. See the PCRE documentation:

PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be recognized.

In the end, this PCRE verb enables . to match any char BUT a CR and LF symbols and $ will match right before either of these two chars.

See more about this and other verbs at rexegg.com:

By default, when PCRE is compiled, you tell it what to consider to be a line break when encountering a . (as the dot it doesn't match line breaks unless in dotall mode), as well the ^ and $ anchors' behavior in multiline mode. You can override this default with the following modifiers:

✽ (*CR) Only a carriage return is considered to be a line break
✽ (*LF) Only a line feed is considered to be a line break (as on Unix)
✽ (*CRLF) Only a carriage return followed by a line feed is considered to be a line break (as on Windows)
✽ (*ANYCRLF) Any of the above three is considered to be a line break
✽ (*ANY) Any Unicode newline sequence is considered to be a line break

For instance, (*CR)w+.w+ matches Line1nLine2 because the dot is able to match the n, which is not considered to be a line break. See demo.