make_clickable()

function make_clickable( $text ) {
	$r               = '';
	$textarr         = preg_split( '/(<[^<>]+>)/', $text, -1, PREG_SPLIT_DELIM_CAPTURE ); // Split out HTML tags.
	$nested_code_pre = 0; // Keep track of how many levels link is nested inside <pre> or <code>.
	foreach ( $textarr as $piece ) {

		if ( preg_match( '|^<code[s>]|i', $piece ) || preg_match( '|^<pre[s>]|i', $piece ) || preg_match( '|^<script[s>]|i', $piece ) || preg_match( '|^<style[s>]|i', $piece ) ) {
			$nested_code_pre++;
		} elseif ( $nested_code_pre && ( '</code>' === strtolower( $piece ) || '</pre>' === strtolower( $piece ) || '</script>' === strtolower( $piece ) || '</style>' === strtolower( $piece ) ) ) {
			$nested_code_pre--;
		}

		if ( $nested_code_pre || empty( $piece ) || ( '<' === $piece[0] && ! preg_match( '|^<s*[w]{1,20}+://|', $piece ) ) ) {
			$r .= $piece;
			continue;
		}

		// Long strings might contain expensive edge cases...
		if ( 10000 < strlen( $piece ) ) {
			// ...break it up.
			foreach ( _split_str_by_whitespace( $piece, 2100 ) as $chunk ) { // 2100: Extra room for scheme and leading and trailing paretheses.
				if ( 2101 < strlen( $chunk ) ) {
					$r .= $chunk; // Too big, no whitespace: bail.
				} else {
					$r .= make_clickable( $chunk );
				}
			}
		} else {
			$ret = " $piece "; // Pad with whitespace to simplify the regexes.

			$url_clickable = '~
				([\s(<.,;:!?])                                # 1: Leading whitespace, or punctuation.
				(                                              # 2: URL.
					[\w]{1,20}+://                                # Scheme and hier-part prefix.
					(?=S{1,2000}s)                               # Limit to URLs less than about 2000 characters long.
					[\w\x80-\xff#%\~/@\[\]*(+=&$-]*+         # Non-punctuation URL character.
					(?:                                            # Unroll the Loop: Only allow puctuation URL character if followed by a non-punctuation URL character.
						['.,;:!?)]                                    # Punctuation URL character.
						[\w\x80-\xff#%\~/@\[\]*(+=&$-]++         # Non-punctuation URL character.
					)*
				)
				()?)                                          # 3: Trailing closing parenthesis (for parethesis balancing post processing).
			~xS';
			// The regex is a non-anchored pattern and does not have a single fixed starting character.
			// Tell PCRE to spend more time optimizing since, when used on a page load, it will probably be used several times.

			$ret = preg_replace_callback( $url_clickable, '_make_url_clickable_cb', $ret );

			$ret = preg_replace_callback( '#([s>])((www|ftp).[w\x80-\xff#$%&~/.-;:=,?@[]+]+)#is', '_make_web_ftp_clickable_cb', $ret );
			$ret = preg_replace_callback( '#([s>])([.0-9a-z_+-]+)@(([0-9a-z-]+.)+[0-9a-z]{2,})#i', '_make_email_clickable_cb', $ret );

			$ret = substr( $ret, 1, -1 ); // Remove our whitespace padding.
			$r  .= $ret;
		}
	}

	// Cleanup of accidental links within links.
	return preg_replace( '#(<a([ rnt]+[^>]+?>|>))<a [^>]+?>([^>]+?)</a></a>#i', '$1$3</a>', $r );
}