SymptomsProvider configured regular expression for checking Tax ID for United Kingdom in PBA-E control panel at Top > Configuration Director > Sales & Financial Settings > Country Specific Settings as follows:
However, when customer enter Tax ID like "GB 111222333" or "GB111222333" the Online Store shows the error message:
Tax Registration ID is incorrect.
The example of Tax ID is correct for used regular expression.
CausePBA-E is using so called 'C regular expressions', while regular expression in the example above was created based on Perl language.
That is why PBA-E Online Store does not accept entered Tax ID, it does not satisfy C regular expressions format.