Missing regex special word boundary escapes

[CAD97]

Specifically, \b{start}, \b{end}, \b{start-half}, \b{end-half}. fancy-regex errors when encountering one of these with InvalidEscape("\\b"). The directional boundary assertions are available as \</\>, but the half-boundary assertions aren't accessible except via lookaround ((?<=\W|\A)/(?=\W|\z)).