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When flex finds a match, yytext points to the first character of the
match in the input buffer. The string itself is part of the input buffer, and
is NOT allocated separately. The value of yytext will be overwritten the next
time yylex() is called. In short, the value of yytext is only valid from within
the matched rule's action.
Often, you want the value of yytext to persist for later processing, i.e., by a parser with non-zero lookahead. In order to preserve yytext, you will have to copy it with strdup() or a similar function. But this introduces some headache because your parser is now responsible for freeing the copy of yytext. If you use a yacc or bison parser, (commonly used with flex), you will discover that the error recovery mechanisms can cause memory to be leaked.
To prevent memory leaks from strdup'd yytext, you will have to track the memory somehow. Our experience has shown that a garbage collection mechanism or a pooled memory mechanism will save you a lot of grief when writing parsers.