Failure handling when Matching optionals

[chichid]

Currently the following expression will always succeed for the B segments...

A = { "a" ~ B* }
B = { "b" }

How can we implement grammar so that the rule is optional, and it only fails when something is provided?

So for the above example:

• a would succeed
• abbbb would succeed
• but a##bb would fail

[CAD97]

The EOI rule requires the End of Input, so { "a" ~ B* ~ EOI } works here as specifically what you asked for.

More generally, you need to use & or ! lookahead; you could use !"#" to require that the following is not #, or &"b" to require that a b follows.

As an example:

WHITESPACE = _{ WHITE_SPACE }

list = { (a | error)* }

error = @{ (!WHITESPACE ~ ANY)+ }

a = @{ "a" ~ (b+ | &(WHITESPACE | EOI)) }
b = @{ "b" }

a abbbb a##bb a

- list
  - a: "a"
  - a: "abbbb"
  - error: "a##bb"
  - a: "a"

[chichid]

thanks @CAD97. The error technic is very interesting stuff!