Ideas for new variables

[dloscutoff]

• Regex matching hex digit: -`[0-9a-f]` (XH)
• Regex matching alphanumeric character: -`[0-9a-z]`
• Regex matching word boundary: `\b` (XB)
• Regex matching single whitespace character: `\s` (XS)
• Regexes matching the opposite of a bunch of common regexes (`\D`, `\W`, `\S`, `\B`, etc.) (maybe a different prefix than X with the same suffixes? Any of K, N, O, Q, or Y are available at the moment... N would be a great mnemonic, although K or Q are less likely to cause collisions with other things we might want to add in the future)