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@@ -0,0 +1,121 @@ +NAME + libparser - Context-free grammar parsing library + +DESCRIPTION + libparser is a small C library that parses input based on a + precompiled context-free grammar. + + To use libparser, a developer should write a syntax for the + input that his application shall parse, in a syntax based + on Extended Backus–Naur form (EBNF) (somewhat simplified but + also somewhat extended). libparser-generate(1) is then used + to create a C source file describing the syntax, which shall + be compiled into an object file with a C compiler. This file + provides a definition of a global variable declared in + <libparser.h>: libparser_rule_table. This variable is used + when calling libparser_parse_file(3) to parse the application's + input. + + libparser is proudly non-self-hosted. + +EXTENDED DESCRIPTION + Syntax + The grammar for libparser-generate(1)'s input can be described + in its own grammar: + + (* CHARACTER CLASSES *) + + _space = " " | "\n" | "\t"; + _alpha = <"a", "z"> | <"A", "Z">; + _octal = <"0", "7">; + _digit = <"0", "9">; + _xdigit = _digit | <"a", "f"> | <"A", "F">; + _nonascii = <128, 255>; + + + (* WHITESPACE/COMMENTS, THE GRAMMAR IS FREE-FORM *) + + _comment_char = _space | <"!", 0x29> | <0x2B, 0xFF>; + _comment_tail = [_comment_char], ("*)" | _comment_tail); + _comment = "(*", _comment_tail; + + _ = {_space | _comment}; + + + (* IDENTIFIERS *) + + _identifier_head = _alpha | _digit | _nonascii | "_"; + _identifier_tail = _identifier_head | "-"; + + identifier = _identifier_head, {_identifier_tail}; + + + (* STRINGS *) + + _escape_simple = "\\" | "\"" | "'" | "a" | "b" | "f" | "n" | "r" | "v"; + _escape_hex = ("x" | "X"), _xdigit, _xdigit; + _escape_octal = _octal, {_octal}; (* May not exceed 255 in base 10 *) + _escape = _escape_simple | _escape_hex | _escape_octal | -; + _character = "\\", _escape | <1, "!"> | <"#", 0xFF>; + + string = "\"", _character, {_character}, "\""; + character = "\"", _character, "\""; + + + (* INTEGERS *) + + _decimal = _digit, {_digit}; + _hexadecimal = "0", ("x" | "X"), _xdigit, {_xdigit}; + + integer = _decimal | _hexadecimal; (* May not exceed 255. *) + + + (* GROUPINGS *) + + _low = character | integer; + _high = character | integer; + + concatenation = _, _operand, _, {",", _, _operand, _}; + alternation = _, concatenation, _, {"|", _, concatenation, _}; + optional = _, "[", _, _expression, _, "]", _; + repeated = _, "{", _, _expression, _, "}", _; + group = _, "(", _, _expression, _, ")", _; + char-range = _, "<", _, _low, _, ",", _, _high, "_", ">", _; + exception = _, "-", _; + embedded-rule = _, identifier, _; + + _literal = char-range | exception | string; + _group = optional | repeated | group | embedded-rule; + _operand = _group | _literal; + + _expression = alternation; + + + (* RULES *) + + rule = _, identifier, _, "=", _, _expression, ";"; + + grammar = {rules, _}, _; + + The file must be encoded in UTF-8, with LF as the line + break (CR and FF are illegal just becuase). + + In alternations, the first (leftmost) match is selected. The + parser is able to backtrack incase it later turns out that it + could not finish that branch. Whenever an exception is + reached, the parser will terminate there. + + Repeated symbols may occour any number of times, including + zero. The compiler is able to backtrack if it takes too much. + + Concatenation has higher precedence than alternation, + groups ("(", ..., ")") have no semantic meaning and are useful + only to put a alternation inside a concatenation without + creating a new rule for that. + + In character ranges, the _high and _low values must be at + least 0 and at most 255, and _high must be greater than _low. + + Rules that begin with an underscore will not show up for + the application in the parse result, the rest of the rules + will appear in the tree-formatted result. |