OpenToken is a facility for performing token analysis and parsing within
the Ada language. It is designed to provide all the functionality of a
traditional lexical analyzer/parser generator, such as lex/yacc. But due
to the magic of inheritance and runtime polymorphism it is implemented
entirely in Ada as withed-in code. No precompilation step is required, and
no messy tool-generated source code is created. The tradeoff is that the
grammar is generated at runtime.

WWW: http://stephe-leake.org/ada/opentoken.html