(?x)^
    (\s*
        (module|class|def
        |unless|if|else|elsif
        |case|when
        |begin|rescue|ensure
        |for|while|until
        |(?= .*? \b(do|begin|case|if|unless)\b )
         # the look-ahead above is to quickly discard non-candidates
         (  "(\\.|[^"])*+"          # 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
     )*
     ( \{ (?!  [^}]*+ \} )
     | \[ (?! [^\]]*+ \] )
     )
    ).*$