[antlr-interest] How to modify this rule?

[chain one]

enumeration_type
: DOT IDENT DOT
;

DOT
: '.'
;
fragment
DIGIT
: '0'..'9'
;

INT
  : '-'? DIGIT+
 ;

FLOAT
:   '.' DIGIT* EXPONENT?
     ;

fragment
EXPONENT: ('e' | 'E') ('+' | '-')? (DIGIT)+;



IDENT

: ('a'..'z'|'A'..'Z'|'_') ('a'..'z'|'A'..'Z'|'_'|'0'..'9')*
;

when the input is ".EXTERNAL." , the lexer don't recognize '.' as a DOT but
try to recognize it as a FLOAT. it will fail after ".E" has been recognized.


How to make the lexer capable of back track? That is also to say if it fails
to recognize a FLOAT, it will try DOT.
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