Problem: Write a Lex program to check if a date is valid or not.
Flex (Fast lexical Analyzer Generator) is a tool/computer program for generating lexical analyzers (scanners or lexers) written by Vern Paxson in C around 1987. Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language. The function yylex() is the main flex function which runs the Rule Section.
Note: Format of Date is DD/MM/YYYY.
Input: 02/05/2019 Output: It is a valid date Input: 05/20/2019 Output: It is not a valid date
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