BigQuery uses re2 for regular expressions, and re2 does support flags.
For example, to do a case insensitive match:
SELECT REGEXP_MATCH('TomatoPotato', r'TOpo')
false
SELECT REGEXP_MATCH('TomatoPotato', r'(?:TOpo)')
false
SELECT REGEXP_MATCH('TomatoPotato', r'(?i:TOpo)')
true