module extension-def {
namespace "urn:simple.extension.typedefs";
prefix "ext";

extension unknown

}

module types {
namespace "urn:custom.types.demo";
prefix "types";

import extension-def

leaf leaf-length-unknown-pattern {
type string

}
}

The given example results in "YangSyntaxErrorException: mismatched input 'pattern' expecting

'" because of the type string restriction order.

type string {
length "2..10"; <--- String restriction
ext:unknown "unknown"; <--- extension
pattern "[0-9a-fA-F]"; <--- String restriction
}

According to the YangParser.g4

type body for string type:
type_body_stmts: (identifier_stmt)* (string_restriction) (identifier_stmt)*;
string_restrictions : (length_stmt |pattern_stmt)*;

Recomended solution:

string_restrictions : (length_stmt | identifier_stmt | pattern_stmt)*;

It will allow any order of restrictions and extensions.