Variable tokens in ANTLR grammar

I am attempting to create a ANTLR grammar for a HL7 derived language. HL7 has a feature that all the delimiters in a message are mapped using the first few bytes of the input itself. For example: MSH|^~\& specifies the various delimiters, in order of field separator | component separator ^, repetition separator ~, escape character \, subcomponent separator &.

Can an ANTLR grammar be produced that does not hardcode these tokens?

As hinted by Kaby76 in the comments: yes, it is possible with some predicate voodoo:

lexer grammar HL7Lexer;

@members {
  private char fieldSeparator;
  private char componentSeparator;
  private char repetitionSeparator;
  private char escapeSeparator;
  private char subcomponentSeparator;
  private boolean separatorsInitialised = false;

  private void setEncodingChars(String chars) {
    this.fieldSeparator = chars.charAt(3);
    this.componentSeparator = chars.charAt(4);
    this.repetitionSeparator = chars.charAt(5);
    this.escapeSeparator = chars.charAt(6);
    this.subcomponentSeparator = chars.charAt(7);
    this.separatorsInitialised = true;
  }

  private boolean isEncodingCharAhead() {
    if (!this.separatorsInitialised) {
      return true;
    }

    char ch = (char)this._input.LA(1);

    return ch == this.fieldSeparator || ch == this.componentSeparator
      || ch == this.repetitionSeparator || ch == this.escapeSeparator
      || ch == this.subcomponentSeparator;
  }
}

MSH
 : 'MSH' . . . . . {this.setEncodingChars(getText());}
 ;

FIELD_SEP
 : {this._input.LA(1) == this.fieldSeparator}? .
 ;

COMPONENT_SEP
 : {this._input.LA(1) == this.componentSeparator}? .
 ;

REPETITION_SEP
 : {this._input.LA(1) == this.repetitionSeparator}? .
 ;

ESCAPE_SEP
 : {this._input.LA(1) == this.escapeSeparator}? .
 ;

SUBCOMPONENT_SEP
 : {this._input.LA(1) == this.subcomponentSeparator}? .
 ;

OTHER
 : ( {!this.isEncodingCharAhead()}? . )+
 ;

When testing this lexer grammar with the input MSH|^~\&|ADT1|GOOD HEALTH HOSPITAL|GHH LAB, INC.|GOOD HEALTH HOSPITAL|198808181126|SECURITY|ADT^A01^ADT_A01|MSG00001|P|2.8||:

String message = "MSH|^~\\&|ADT1|GOOD HEALTH HOSPITAL|GHH LAB, INC.|GOOD HEALTH HOSPITAL|198808181126|SECURITY|ADT^A01^ADT_A01|MSG00001|P|2.8||";
HL7Lexer lexer = new HL7Lexer(CharStreams.fromString(message));

CommonTokenStream stream = new CommonTokenStream(lexer);

stream.fill();

for (Token t : stream.getTokens()) {
    System.out.printf("%-20s '%s'\n",
        HL7Lexer.VOCABULARY.getSymbolicName(t.getType()),
        t.getText().replace("\n", "\\n"));
}

the following tokens are created:

MSH                  'MSH|^~\&'
FIELD_SEP            '|'
OTHER                'ADT1'
FIELD_SEP            '|'
OTHER                'GOOD HEALTH HOSPITAL'
FIELD_SEP            '|'
OTHER                'GHH LAB, INC.'
FIELD_SEP            '|'
OTHER                'GOOD HEALTH HOSPITAL'
FIELD_SEP            '|'
OTHER                '198808181126'
FIELD_SEP            '|'
OTHER                'SECURITY'
FIELD_SEP            '|'
OTHER                'ADT'
COMPONENT_SEP        '^'
OTHER                'A01'
COMPONENT_SEP        '^'
OTHER                'ADT_A01'
FIELD_SEP            '|'
OTHER                'MSG00001'
FIELD_SEP            '|'
OTHER                'P'
FIELD_SEP            '|'
OTHER                '2.8'
FIELD_SEP            '|'
FIELD_SEP            '|'
EOF                  '<EOF>'