Question

I’m trying to match a control character in the form \^c where c is any valid character for control characters. I have this regular expression, but it’s not currently working: \\[^][@-z]

I think the problem lies with the fact that the caret character (^) is part of the regular expressions parsing engine.

Answer 1

Match an ASCII text string of the form ^X using the pattern \^., nothing more. Match an ASCII text string of the form \^X with the pattern \\\^.. You may wish to constrain that dot to [?@_\[\]^\\], so \\\^[A-Z?@_\[\]^\\]. It’s easier to read as [?\x40-\x5F] for the bracketed character class, hence \\\^[?\x40-\x5F] for a literal BACKSLASH, followed by a literal CIRCUMFLEX, followed by something that turns into one of the valid control characters.

Note that that is the result of printing out the pattern, or what you’d read from a file. It’s what you need to pass to the regex compiler. If you have it as a string literal, you must of course double each of those backslashes. `\\\\\\^[?\\x40-\\x5F]" Yes, it is insane looking, but that is because Java does not support regexes directly as Groovy and Scala — or Perl and Ruby — do. Regex work is always easier without the extra bbaacckksslllllaasshheesssssess. 🙂

If you had real control characters instead of indirect representations of them, you would use \pC for all literal code points with the property GC=Other, or \p{Cc} for just GC=Control.