hexadecimal value escapes

These are currently supported by GoAWK:

$ goawk 'BEGIN { print "hello\x21" }'
hello!

However they are not defined by POSIX:

One sequence that is not supported is hexadecimal value escapes beginning with
\x. This would allow values expressed in more than 9 bits to be used within
awk as in the ISO C standard. However, because this syntax has a
non-deterministic length, it does not permit the subsequent character to be a
hexadecimal digit. This limitation can be dealt with in the C language by the
use of lexical string concatenation. In the awk language, concatenation
could also be a solution for strings, but not for extended regular expressions
(either lexical ERE tokens or strings used dynamically as regular
expressions). Because of this limitation, the feature has not been added to
POSIX.1-2017.

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/awk.html#tag_20_06_18

Gawk deals with this by disabling them via gawk --posix or
POSIXLY_CORRECT=y. If the feature is to remain it should perhaps be mentioned
here:

https://github.com/benhoyt/goawk#differences-from-awk

[benhoyt]

Thanks for the note. I think I'm comfortable leaving this as is, because it's the default behavior of awk (Brian Kernighan's original "one true awk"). In several places GoAWK adds features that are in awk but not in the POSIX spec. Note that GoAWK doesn't allow \x forms in /regex/ tokens.