Newbie: 2.4# is accepted as a float. Is '#' a special character? - antlr4

Question

Wondering why the expression "setvalue(2#)' is happily accepted by the lexer (and parser) given my grammar/visitor. I am sure I am doing something wrong.
Below is a small sample that should illustrate the problem.
Any help is much appreciated.
grammar ExpressionEvaluator;
parse
: block EOF
;
block
: stat*
;
stat
: assignment
;
assignment
: SETVALUE OPAR expr CPAR
;
expr
: atom #atomExpr
;
atom
: OPAR expr CPAR #parExpr
| (INT | FLOAT) #numberAtom
| ID #idAtom
;
OPAR : '(';
CPAR : ')';
SETVALUE : 'setvalue';
ID
: [a-zA-Z_] [a-zA-Z_0-9]*
;
INT
: [0-9]+
;
FLOAT
: [0-9]+ '.' [0-9]*
| '.' [0-9]+
;
STRING
: '"' (~["\r\n] | '""')* '"'
;
SPACE
: [ \t\r\n] -> skip
;
Code snippet:
public override object VisitParse(ExpressionEvaluatorParser.ParseContext context)
{
return this.Visit(context.block());
}
public override object VisitAssignment(ExpressionEvaluatorParser.AssignmentContext context)
{
// TODO - Set ID Value
return Convert.ToDouble(this.Visit(context.expr()));
}
public override object VisitIdAtom(ExpressionEvaluatorParser.IdAtomContext context)
{
string id = context.GetText();
// TODO - Lookup ID value
return id;
}
public override object VisitNumberAtom(ExpressionEvaluatorParser.NumberAtomContext context)
{
return Convert.ToDouble(context.GetText());
}
public override object VisitParExpr(ExpressionEvaluatorParser.ParExprContext context)
{
return this.Visit(context.expr());
}

Answer 1

The # character actually isn't matching anything at all. When the lexer reaches that character, the following happen in order:
The lexer determines that no lexer rule can match the # character.
The lexer reports an error regarding the failure.
The lexer calls _input.consume() to skip past the bad character.
To ensure errors are reported as easily as possible, always add the following rule as the last rule in your lexer.
ErrChar
: .
;
The parser will report an error when it reaches an ErrChar, so you won't need to add an error listener to the lexer.