MySQL REGEXP does not work properly with Russian chars - mysql

Question

SELECT 'Hello' REGEXP '^[^aeiouAEIOU][A-Za-z]*$' -> 1
SELECT 'Привет' REGEXP '^[^аеиоуыэюяАЕИОУЫЭЮЯ][А-Яа-я]*$' -> 0 - it must return 1.

Answer 1

MySQL's REGEXP only works with bytes. Russian characters are each 2 bytes.
For limiting to Cyrillic, this seems to be correct:
SELECT HEX('Привет') REGEXP '^((D0|D1)..)+$'; -- > 1
(I'll get to the issue of avoiding leading vowels in a minute.)
To explain:
All Russian characters are 2 bytes, the first byte being hex D0 or D1. (There may be non-Russian characters starting that way; I'm ignoring that problem.)
(...|...) - | means 'OR'.
.. matches a 2-byte hex, saying that the second byte can be anything (this is overkill but may not hurt).
(...)+ - The plus means one or more occurrence.
The ^ and $ 'anchor' the regexp to encompass the entire string.
Back to the no-leading-vowel issue. Now we need to play some painful games to list the vowels; their HEX seem to be
D0, followed by any of B0 B5 B8 BE
90 95 98 9E, or
D1, followed by any of 83 8B 8D 8E 8F
A3 AB AD AE AF
Example: select hex('э'); --> D18D.
Putting it all together will be messy because MySQL does not have the (? tools to say "not". So, I will start by testing for a leading vowel:
SELECT HEX('Привет')
REGEXP '^(D0(B0|B5|B8|BE|90|95|98|9E))|(D1(83|8B|8D|8E|8F|A3|AB|AD|AE|AF))'
correctly fails.
Now to put things together:
SELECT NOT HEX('Привет')
REGEXP '^(D0(B0|B5|B8|BE|90|95|98|9E))|(D1(83|8B|8D|8E|8F|A3|AB|AD|AE|AF))'
AND HEX('Привет')
REGEXP '^((D0|D1)..)+$';
The first part check for NOT a leading vowel; the second part check for all characters being Russian.
That test case works, and 'э' came back with 0, but I may have goofed somewhere.
(That was a challenge.)