Useful Regex I could use someday...

regex.regex

Matching href without " or ' (with escaping support):
(?<=href\=('|"))((?!\1).)+

Matching youtube video URLs without " or ' (with escaping support)
(?<=href\=('|"))(((http(s)?:)?//)?(youtube.com))?/watch\?((?!\1).)+

Matching any >1 consecutive empty lines:
(?<=\s\n)(\s*\n)

Matching per line slash (/) separated values without slashes
([^/\r\n]+)

Matching <% ... %> code blocks in Mako templates
<%(.*?\n?)+?%> and the less memory killer (sublime text): (?s)<%.+?%> (flag s for multiline)