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Lexical
analysis reads characters from left to right and groups into tokens. A simple
way to build lexical analyzer is to construct a diagram to illustrate the
structure of tokens of the source program. We can also produce a lexical
analyzer automatically by specifying the lexeme patterns to a lexical-analyzer
generator and compiling those patterns into code that functions
as a lexical analyzer. This approach makes it easier to modify a lexical analyzer,
since we have only to rewrite the affected patterns, not the entire program.
Three
general approaches for implementing lexical analyzer are:
i.
Use lexical analyzer generator (LEX) from a regular expression based
specification that provides routines for reading and buffering the input.
ii.
Write lexical analyzer in conventional language using I/O facilities to read
input.
iii.
Write lexical analyzer in assembly language and explicitly manage the reading
of input.
The
speed of lexical analysis is a concern in compiler design, since only this
phase reads the source program character-by character.
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