(?x)^
    (\s*
        (module|class|def
        |if|else|elsif
        |case|when
        |begin|rescue
        |for|while|unless
        |(?= .*? \b(do|begin|case|if|unless)\b )
         (  "(\\.|[^"])*+"			# eat a double quoted string
         | ''(\\.|[^''])*+''		# eat a single quoted string
         |   [^#"'']				# eat all but comments and strings
         )*
         (                 \s   (do|begin|case)
         | [-+=&|*/~%^<>~] \s*+ (if|unless)
         )
        )\b
        (?! [^;]*+ ; .*? \bend\b )
    |(  "(\\.|[^"])*+"              # eat a double quoted string
     | ''(\\.|[^''])*+''            # eat a single quoted string
     |   [^#"'']                    # eat all but comments and strings
     )*
     \{ (?! [^}]*+ \} )
    ).*$