yyleng

yyleng is a variable used by the Lex/Flex family of lexical analyzer generators to store the length of the text matched by the current token. In typical Flex-generated scanners, yyleng corresponds to the number of characters in the string pointed to by yytext, which contains the actual token that was matched. The value of yyleng is updated automatically for each successful match and can be referenced within action code to process the token without relying on a separate NUL-terminated string.

Usage and practical notes:

- yyleng is commonly used in action blocks to examine or copy only the relevant portion of yytext, for example when extracting a token’s value or performing length-based decisions.

- It is closely tied to yytext, since its value reflects the length of the data stored there.

- The exact declaration (type and scope) of yyleng can depend on the skeleton (C, C++, etc.) and the Flex version, but it is generally considered an integer-like variable provided by the generated scanner.

Origin and context:

- The term originates from the Unix toolset Lex and its modern reimplementation Flex. yyleng is one of several built-in variables (such as yytext and yylineno) that facilitate token handling in lexical analysis.

- While primarily associated with Flex, similar concepts exist in other lexer generators, sometimes under different names.

See also:

- yytext, Flex, Lex, lexer, token, tokenization.