Grouping

Scripts > Productions > Grouping

By means of the parenthesis '(' and ')' expressions can be grouped.

So the preference of concatenation can be eliminated.

Example:

("a" | "b")  ("c" | "d")        matches        "a c", "a d", "b c" and "b d"

"a" | "b" "c" | "d"           matches        "a",  "b c" and "d"

If you forget the preference of concatenation, this can lead to astonishing results, especially, if semantic actions are involved.

Example:

{{str s;}}

"a"

{{s = "a"; }}

| "b"

{{s = "b"; }}

will result in an error message for the last line: Unknown identifier: s

Implicitly the first three lines are a closed chain. Only inside of this scope the declaration of string s is valid. Correctly you have to write:

{{str s;}}

(

"a"

{{s = "a"; }}

| "b"

{{s = "b"; }}

)

A different example for this is the Inner-production in the introduction.  There parenthesis around "b" | "c" are necessary.

This page belongs to the TextTransformer Documentation