Here is a PHP function with a commented regex that matches each of these URL forms and converts them to links (if they are not links already):
// Linkify youtube URLs which are not already links.function linkifyYouTubeURLs($text) { $text = preg_replace('~
# Match non-linked youtube URL in the wild. (Rev:20111012)
https?:// # Required scheme. Either http or https.
(?:[0-9A-Z-]+\.)? # Optional subdomain.
(?: # Group host alternatives.
youtu\.be/ # Either youtu.be,
| youtube\.com # or youtube.com followed by
\S* # Allow anything up to VIDEO_ID,
[^\w\-\s] # but char before ID is non-ID char.
) # End host alternatives.
([\w\-]{11}) # $1: VIDEO_ID is exactly 11 chars.
(?=[^\w\-]|$) # Assert next char is non-ID or EOS.
(?! # Assert URL is not pre-linked.
[?=&+%\w]* # Allow URL (query) remainder.
(?: # Group pre-linked alternatives.
[\'"][^<>]*> # Either inside a start tag,
| </a> # or inside <a> element text contents.
) # End recognized pre-linked alts.
) # End negative lookahead assertion.
[?=&+%\w-]* # Consume any URL (query) remainder.
~ix',
'<a href="http://www.youtube.com/watch?v=$1">YouTube link: $1</a>', $text); return $text;}
And Here is a JavaScript version with the exact same regex (with comments removed):
// Linkify youtube URLs which are not already links.function linkifyYouTubeURLs(text) { var re = /https?:\/\/(?:[0-9A-Z-]+\.)?(?:youtu\.be\/|youtube\.com\S*[^\w\-\s])([\w\-]{11})(?=[^\w\-]|$)(?![?=&+%\w]*(?:['"][^<>]*>|<\/a>))[?=&+%\w-]*/ig; return text.replace(re, '<a href="http://www.youtube.com/watch?v=$1">YouTube link: $1</a>');}
Notes:
The VIDEO_ID portion of the URL is captured in the one and only capture group: $1.
If you know that your text does not contain any pre-linked URLs, you can safely remove the negative lookahead assertion which tests for this condition (The assertion beginning with the comment:"Assert URL is not pre-linked.") This will speed up the regex somewhat.
The replace string can be modified to suit. The one provided above simply creates a link to the generic "http://www.youtube.com/watch?v=VIDEO_ID" style URL and sets the link text to:"YouTube link: VIDEO_ID".
추천 : 
