flex default rule

Dimitri Maziuk <dmaziuk@bmrb.wisc.edu>
1 Apr 2006 20:49:58 -0500

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From: Dimitri Maziuk <dmaziuk@bmrb.wisc.edu>
Newsgroups: comp.compilers
Date: 1 Apr 2006 20:49:58 -0500
Organization: only myself
Keywords: lex, question

Hi all,


here's what I can't quite figure out:


Rule: multi-line text values are enclosed in "\n;"


Relevant flex specs:


NL (\r|\n|\r\n)
WS [ \t\r\n\b\012]
SEMI ^;
%x YYSEMI
%option c++
%debug


%%
<INITIAL>{SEMI} {
    cerr "Entering YYSEMI" << endl;
    BEGIN YYSEMI;
}
<YYSEMI>{SEMI} {
    cout << buf.c_str() << endl; // buf is an std::string
    BEGIN INITIAL;
}
<INITIAL>[^{SEMI}] {
    cerr << "in YYSEMI, token" << yytext << endl;
    buf.append( yytext );
}


Input:
;
Solution Structure and Backbone Dynamics of Component IV
Glycera dibranchiata Monomeric Hemoglobin-CO
;


Final output (from second rule):


olution tructure and Backbone Dynamics of Component V
Glycera dibranchiata onomeric Hemoglobin-CO


Trace output:


--accepting rule at line 114(";")
Entering YYSEMI
--accepting rule at line 189("
")
in YYSEMI, token:


--(end of buffer or a NUL)
--accepting default rule ("S")
--(end of buffer or a NUL)
S--accepting rule at line 189("o")
in YYSEMI, token: o
...
--accepting rule at line 189("n")
in YYSEMI, token: n
--(end of buffer or a NUL)
--accepting rule at line 189(" ")
in YYSEMI, token:
--(end of buffer or a NUL)
--accepting default rule ("S")
--(end of buffer or a NUL)
S--accepting rule at line 189("t")
in YYSEMI, token: t
--(end of buffer or a NUL)
...
--accepting rule at line 189("t")
in YYSEMI, token: t
--(end of buffer or a NUL)
--accepting rule at line 189(" ")
in YYSEMI, token:
--(end of buffer or a NUL)
--accepting default rule ("I")
--(end of buffer or a NUL)
I--accepting rule at line 189("V")
in YYSEMI, token: V
...


and so on.


Why the hell does it trigger the default rule at 'S', 'S', 'I', and 'M'?
Any takers?


TIA
Dima


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