lark/examples/conf.py

#
# This example demonstrates the power of the contextual lexer, by parsing a config file.
#
# The tokens NAME and VALUE match the same input. A regular lexer would arbitrarily
# choose one over the other, which would lead to a (confusing) parse error.
# However, due to the unambiguous structure of the grammar, the LALR(1) algorithm knows
# which one of them to expect at each point during the parse.
# The lexer then only matches the tokens that the parser expects.
# The result is a correct parse, something that is impossible with a regular lexer.
#
# Another approach is to discard a lexer altogether and use the Earley algorithm.
# It will handle more cases than the contextual lexer, but at the cost of performance.
# See examples/conf_nolex.py for an example of that approach.
#

from lark import Lark

parser = Lark(r"""
        start: _NL? section+
        section: "[" NAME "]" _NL item+
        item: NAME "=" VALUE _NL
        NAME: /[a-zA-Z_]\w*/
        VALUE: /.*/

        WS.ignore: /[\t \f]+/
        COMMENT.ignore: /\#[^\n]*/
        _NL: /(\r?\n)+/
    """, parser="lalr_contextual_lexer")


sample_conf = """
[bla]
a=Hello
this="that",4
"""

print(parser.parse(sample_conf).pretty())