lex & yacc, 2nd Edition

Glossary

A large number of technical terms are used in this manual. Many of them may be familiar; some may not. To help avoid confusion, the most significant terms are listed here.

action

The C code associated with a lex pattern or a yacc rule. When the pattern or rule matches an input sequence, the action code is executed.

alphabet

A set of distinct symbols. For example, the ASCII character set is a collection of 128 different symbols. In a lex specification, the alphabet is the native character set of the computer, unless you use “%T” to define a custom alphabet. In a yacc grammar, the alphabet is the set of tokens and non-terminals used in the grammar.

ambiguity

An ambiguous grammar is one with more than one rule or set of rules that match the same input. In a yacc grammar, ambiguous rules cause shift/reduce or reduce/reduce conflicts. The parsing mechanism that yacc uses cannot handle ambiguous grammars, so it uses %prec declarations and its own internal rules to resolve the conflict when creating a parser.

Lex specifications can be and usually are ambiguous; when two patterns match the same input, the pattern earlier in the specification wins.

ASCII

American Standard Code for Information Interchange; a collection of 128 symbols representing the common symbols found in the American alphabet: lower and uppercase letters, digits, and punctuation, plus additional characters for formatting and control of data communication links. Most computers on which yacc and lex run use ASCII, although ...