Dec 28, 2006 This is an online compiler for your flex applications.Without even installing a single component in your pc, you can just copy paste your code into the code area of this very site and see your swf will be created online. Give it a try, you will love it. LEX: Lex - A Lexical Analyzer Generator M. Lex helps write programs whose control flow is directed by instances of regular expressions in the input stream. It is well suited for editor-script type transformations and for segmenting input in preparation for a parsing routine.

The Lex & Yacc Page

Lex Compiler Editor Online Python

M. E. Lesk and E. Schmidt

ABSTRACT

Lex helps write programs whose control flow is directed by instances of regular expressions in the input stream. It is well suited for editor-script type transformations and for segmenting input in preparation for a parsing routine.

Lex source is a table of regular expressions and corresponding program fragments. The table is translated to a program which reads an input stream, copying it to an output stream and partitioning the input into strings which match the given expressions. As each such string is recognized the corresponding program fragment is executed. The recognition of the expressions is performed by a deterministic finite automaton generated by Lex. The program fragments written by the user are executed in the order in which the corresponding regular expressions occur in the input stream.

The lexical analysis programs written with Lex accept ambiguous specifications and choose the longest match possible at each input point. If necessary, substantial lookahead is performed on the input, but the input stream will be backed up to the end of the current partition, so that the user has general freedom to manipulate it.

Lex can generate analyzers in either C or Ratfor, a language which can be translated automatically to portable Fortran. It is available on the PDP-11 UNIX, Honeywell GCOS, and IBM OS systems. This manual, however, will only discuss generating analyzers in C on the UNIX system, which is the only supported form of Lex under UNIX Version 7. Lex is designed to simplify interfacing with Yacc, for those with access to this compiler-compiler system.

1. Introduction.

Lex is a program generator designed for lexical processing ofcharacter input streams. It accepts a high-level, problemoriented specification for character string matching, and producesa program in a general purpose language which recognizes regularexpressions. The regular expressions are specified by the user inthe source specifications given to Lex. The Lex written coderecognizes these expressions in an input stream and partitions theinput stream into strings matching the expressions. At theboundaries between strings program sections provided by the userare executed. The Lex source file associates the regularexpressions and the program fragments. As each expression appearsin the input to the program written by Lex, the correspondingfragment is executed.

The user supplies the additional code beyond expressionmatching needed to complete his tasks, possibly including codewritten by other generators. The program that recognizes theexpressions is generated in the general purpose programminglanguage employed for the user's program fragments. Thus, a highlevel expression language is provided to write the stringexpressions to be matched while the user's freedom to writeactions is unimpaired. This avoids forcing the user who wishes touse a string manipulation language for input analysis to writeprocessing programs in the same and often inappropriate stringhandling language.

Lex is not a complete language, but rather a generatorrepresenting a new language feature which can be added todifferent programming languages, called ``host languages.' Justas general purpose languages can produce code to run on differentcomputer hardware, Lex can write code in different host languages.The host language is used for the output code generated by Lex andalso for the program fragments added by the user. Compatiblerun-time libraries for the different host languages are alsoprovided. This makes Lex adaptable to different environments anddifferent users. Each application may be directed to thecombination of hardware and host language appropriate to the task,the user's background, and the properties of local implementations.At present, the only supported host language is C,although Fortran (in the form of Ratfor [2] has been available inthe past. Lex itself exists on UNIX, GCOS, and OS/370; but thecode generated by Lex may be taken anywhere the appropriatecompilers exist.

Lex turns the user's expressions and actions (called sourcein this memo) into the host general-purpose language; thegenerated program is named yylex. The yylex program willrecognize expressions in a stream (called input in this memo) andperform the specified actions for each expression as it isdetected. See Figure 1. For a trivial example, consider a program to delete from theinput all blanks or tabs at the ends of lines.is all that is required. The program contains a %% delimiter tomark the beginning of the rules, and one rule. This rule containsa regular expression which matches one or more instances of thecharacters blank or tab (written t for visibility, in accordancewith the C language convention) just prior to the end of a line.The brackets indicate the character class made of blank and tab;the + indicates ``one or more ..'; and the $ indicates ``endof line,' as in QED. No action is specified, so the programgenerated by Lex (yylex) will ignore these characters. Everythingelse will be copied. To change any remaining string of blanks ortabs to a single blank, add another rule:The finite automaton generated for this source will scan for bothrules at once, observing at the termination of the string ofblanks or tabs whether or not there is a newline character, andexecuting the desired rule action. The first rule matches allstrings of blanks or tabs at the end of lines, and the second ruleall remaining strings of blanks or tabs.

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Lex can be used alone for simple transformations, or foranalysis and statistics gathering on a lexical level. Lex canalso be used with a parser generator to perform the lexicalanalysis phase; it is particularly easy to interface Lex and Yacc[3]. Lex programs recognize only regular expressions; Yacc writesparsers that accept a large class of context free grammars, butrequire a lower level analyzer to recognize input tokens. Thus, acombination of Lex and Yacc is often appropriate. When used asa preprocessor for a later parser generator, Lex is used topartition the input stream, and the parser generator assignsstructure to the resulting pieces. The flow of control in sucha case (which might be the first half of a compiler, for example)is shown in Figure 2. Additional programs, written by othergenerators or by hand, can be added easily to programs written byLex.Yacc users will realize that the name yylex is what Yacc expectsits lexical analyzer to be named, so that the use of this name byLex simplifies interfacing.

Lex generates a deterministic fin