regexp: Add test for subexpression match
regexp: Add test for subexpression match

Closes #497

P71/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,joshuarubin/dotfiles,joshuarubin/dotfiles,phy1729/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,joshuarubin/dotfiles,joshuarubin/dotfiles,stuha/wbr-zsh,x-way/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting