From the anguish of those who try to implement the POSIX shell, I understand that edge cases are far from difficult to find. The Almquist shell (Debian Dash) is cited for not using lex/yacc to implement the parser, and described as untrustworthy.

https://archive.fosdem.org/2018/schedule/event/code_parsing_...

A major goal of this later ADA implementation is readability, in addition to (Windows) performance and ADA's safety.

https://archive.fosdem.org/2019/schedule/event/ada_shell/