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Lex Programming Language Help For Students

Introduction to Lex

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.

Example: Structure of a Lex ?le:

&int main()



printf("There were %d characters in %d lines\n",


return 0;


If you store this code in a ?le count.l, you can build an executable from it by

lex -t count.l > count.c

cc -c -o count.o count.l

cc -o counter count.o -ll

1You see that the lex ?le is ?rst turned into a n

Compiler Download

To get started, see Download and install RPL and follow the instructions.



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