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FAQ

From time to time, the flex maintainer receives certain questions. Rather than repeat answers to well-understood problems, we publish them here.

• When was flex born?
• How do I expand backslash-escape sequences in C-style quoted strings?
• Why do flex scanners call fileno if it is not ANSI compatible?
• Does flex support recursive pattern definitions?
• How do I skip huge chunks of input (tens of megabytes) while using flex?
• Flex is not matching my patterns in the same order that I defined them.
• My actions are executing out of order or sometimes not at all.
• How can I have multiple input sources feed into the same scanner at the same time?
• Can I build nested parsers that work with the same input file?
• How can I match text only at the end of a file?
• How can I make REJECT cascade across start condition boundaries?
• Why can’t I use fast or full tables with interactive mode?
• How much faster is -F or -f than -C?
• If I have a simple grammar can’t I just parse it with flex?
• Why doesn’t yyrestart() set the start state back to INITIAL?
• How can I match C-style comments?
• The ’.’ isn’t working the way I expected.
• Can I get the flex manual in another format?
• Does there exist a "faster" NDFA->DFA algorithm?
• How does flex compile the DFA so quickly?
• How can I use more than 8192 rules?
• How do I abandon a file in the middle of a scan and switch to a new file?
• How do I execute code only during initialization (only before the first scan)?
• How do I execute code at termination?
• Where else can I find help?
• Can I include comments in the "rules" section of the file?
• I get an error about undefined yywrap().
• How can I change the matching pattern at run time?
• How can I expand macros in the input?
• How can I build a two-pass scanner?
• How do I match any string not matched in the preceding rules?
• I am trying to port code from AT&T lex that uses yysptr and yysbuf.
• Is there a way to make flex treat NULL like a regular character?
• Whenever flex can not match the input it says "flex scanner jammed".
• Why doesn’t flex have non-greedy operators like perl does?
• Memory leak - 16386 bytes allocated by malloc.
• How do I track the byte offset for lseek()?
• How do I use my own I/O classes in a C++ scanner?
• How do I skip as many chars as possible?
• deleteme00
• Are certain equivalent patterns faster than others?
• Is backing up a big deal?
• Can I fake multi-byte character support?
• deleteme01
• Can you discuss some flex internals?
• unput() messes up yy_at_bol
• The | operator is not doing what I want
• Why can’t flex understand this variable trailing context pattern?
• The ^ operator isn’t working
• Trailing context is getting confused with trailing optional patterns
• Is flex GNU or not?
• ERASEME53
• I need to scan if-then-else blocks and while loops
• ERASEME55
• ERASEME56
• ERASEME57
• Is there a repository for flex scanners?
• How can I conditionally compile or preprocess my flex input file?
• Where can I find grammars for lex and yacc?
• I get an end-of-buffer message for each character scanned.
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• What is the difference between YYLEX_PARAM and YY_DECL?
• Why do I get "conflicting types for yylex" error?
• How do I access the values set in a Flex action from within a Bison action?