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A Flex lexical analyzer usually has time complexity () in the length of the input. That is, it performs a constant number of operations for each input symbol. This constant is quite low: GCC generates 12 instructions for the DFA match loop.
Lexical analysis mainly segments the input stream of characters into tokens, simply grouping the characters into pieces and categorizing them. However, the lexing may be significantly more complex; most simply, lexers may omit tokens or insert added tokens.
The RE/flex lexical analyzer generator accepts an extended syntax of Flex lexer specifications as input. The RE/flex specification syntax is more expressive than the traditional Flex lexer specification syntax and may include indentation anchors, word boundaries, lazy quantifiers (non-greedy, lazy repeats), and new actions such as wstr() to ...
Flex does not "rescan" the input when the buffer grows. And even if it did (it doesn't) the amortized time analysis above is incorrect. It'd be not because there would be (/) resizes. But again, it does not rescan when the buffer grows.
A compiler is likely to perform some or all of the following operations, often called phases: preprocessing, lexical analysis, parsing, semantic analysis (syntax-directed translation), conversion of input programs to an intermediate representation, code optimization and machine specific code generation.
[1] [2] It is commonly used with the yacc parser generator and is the standard lexical analyzer generator on many Unix and Unix-like systems. An equivalent tool is specified as part of the POSIX standard. [3] Lex reads an input stream specifying the lexical analyzer and writes source code which implements the lexical analyzer in the C ...
Flex, an automatic lexical analyser, is often used with Bison, to tokenise input data and provide Bison with tokens. [5] Bison was originally written by Robert Corbett in 1985. [1] Later, in 1989, Robert Corbett released another parser generator named Berkeley Yacc. Bison was made Yacc-compatible by Richard Stallman. [6]
Input strings are generated so that they extensively cover DFA transitions and paths. Data generation happens right after DFA construction and prior to any optimizations, but the lexer itself is fully optimized, so skeleton programs are capable of revealing any errors in optimizations and code generation.