PHP validation/regex for URL

Use the filter_var() function to validate whether a string is URL or not:

var_dump(filter_var('', FILTER_VALIDATE_URL));

It is bad practice to use regular expressions when not necessary.

EDIT: Be careful, this solution is not unicode-safe and not XSS-safe. If you need a complex validation, maybe it's better to look somewhere else.

I used this on a few projects, I don't believe I've run into issues, but I'm sure it's not exhaustive:

$text = preg_replace(
  "'<a href=\"$1\" target=\"_blank\">$3</a>$4'",

Most of the random junk at the end is to deal with situations like http://domain.example. in a sentence (to avoid matching the trailing period). I'm sure it could be cleaned up but since it worked. I've more or less just copied it over from project to project.