Lexical Analysis With Flex, for Flex 2.6.4: FAQ

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 cant I use fast or full tables with interactive mode?:
• How much faster is -F or -f than -C?:
• If I have a simple grammar cant 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 period 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:
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• 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?: