[PATCH 2] Port some QRegExp usage to QRegularExpression

src/common.*

Octal escape sequences with more than three digits \nnn, must be wrapped in
\o{....}, c.f. https://perldoc.perl.org/perlre

Use QRegularExpression::anchoredPattern() to get similar behaviour to
QRegExp::exactMatch().

colorOnlyRegExp isn't used anywhere AFAICS.