qdoc: Remove uses of QRegular Expression

This change replaces two uses of QRegularExpression
with a simple ad hoc parser. qdoc is not allowed to
use QRegularExpression because it is not in the
bootstrap library.

Task-number: QTBUG-62845
Change-Id: Icfaca2181ff1879f02d111626a85fb3da758c731
Reviewed-by: Martin Smith <martin.smith@qt.io>