Queries with Regular Expressions

Hello!

My queries work fine with Regular Expressions, as:

SELECT field FROM table WHERE field ~ 'something';
SELECT field FROM table WHERE field ~* 'something';
SELECT field FROM table WHERE field ~* 'som[i,e]thing';

And it works fine for special characters:
SELECT field FROM table WHERE field ~* 'chão';

But I just can't make it work correctly using brackets:
SELECT field FROM table WHERE field ~* 'ch[aã]o';

It just returns tuples that have 'chao', but not 'chão'.

My queries are utf-8 an the database is SQL_ASCII.

Any idea? Thank you!

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Silas Justiniano - Brazil