For regex information see the manual:

• PCRE Pattern modifiers
• PCRE Pattern syntax
• POSIX regex man pages

Some information about the regex tool:

• Click on - and + to hide and display that box. Hiding settings are stored in a cookie.
• To highlight the match in the subject, click on the number in the "P" column.
• Highlighting does not work, if positions are not available or full subject is disabled.
• Use the letters on top right corner for quick navigation
• See regex tool manual for full help