[libiodata] Fix regexp used for parsing hex values

The function iodata::output::prepare(...) [1] replaces certain characters with hex values, like \x20 for a quotation mark. The function is called when data is saved to a file, see iodata::output::output_record(...).

However, the regexp used for reading saved data from a file looked for hex values encoded as \x{20}, which is incorrect, causing hex values of form \x20 be interpreted as stray backslashes instead, which aborts the parsing process.

[1] https://github.com/pgerdt/libiodata/blob/master/src/validator.cpp#L488