Antlr4 Grammar/Rules - issue with solving BASIC print variable - antlr4

Question

The scenario is that I want to create a BASIC (high level) language using ANTRL4.
The test input below is the creation of a variable called C$ and assigning an integer value. The value assignment works. The print statement works except where concatenating the variable to it:-
************ TEST CASE ****************
$C=15;
print "dangerdanger!"; # print works
print "Number of GB left=" + $C;
Using a Parse Tree Inspector I can see assignments are working fine but when it gets to the identification of the variable in the string it seems there is a mismatched input '+' expecting STMTEND.
I wondered if anyone could help me out here and see what adjustment I need to make to my rules and grammar to solve this issue.
Many thanks in advance.
Kevin
PS. As a side issue I would rather have C$ than $C but early days...
********RULES************
VARNAME : '$'('A'..'Z')*
;
CONCAT : '+'
;
STMTEND : SEMICOLON NEWLINE* | NEWLINE+
;
STRING : SQUOTED_STRING (CONCAT SQUOTED_STRING | CONCAT VARNAME)*
| DQUOTED_STRING (CONCAT DQUOTED_STRING | CONCAT VARNAME)*
;
fragment SQUOTED_STRING : '\'' (~['])* '\''
;
fragment DQUOTED_STRING
: '"' ( ESC_SEQ| ~('\\'|'"') )* '"'
;
fragment ESC_SEQ
: '\\' ('b'|'t'|'n'|'f'|'r'|'\"'|'\''|'\\')
| UNICODE_ESC
| OCTAL_ESC
;
fragment OCTAL_ESC
: '\\' ('0'..'3') ('0'..'7') ('0'..'7')
| '\\' ('0'..'7') ('0'..'7')
| '\\' ('0'..'7')
;
fragment HEX_DIGIT : '0x' ('0'..'9' | 'a'..'f' | 'A'..'F')+
;
fragment UNICODE_ESC : '\\' 'u' HEX_DIGIT HEX_DIGIT HEX_DIGIT HEX_DIGIT
;
SEMICOLON : ';'
;
NEWLINE : '\r'?'\n'
************GRAMMAR************
print_command
: PRINT STRING STMTEND #printCommandLabel
;
assignment
: VARNAME EQUALS INTEGER STMTEND #assignInteger
| VARNAME EQUALS STRING STMTEND #assignString
;

Answer 1

You shouldn't try to create concat-expressions inside your lexer: that is the responsibility of the parser. Something like this should do it:
print_command
: PRINT STRING STMTEND #printCommandLabel
;
assignment
: VARNAME EQUALS expression STMTEND
;
expression
: expression CONCAT expression
| INTEGER
| STRING
| VARNAME
;
CONCAT
: '+'
;
VARNAME
: '$'('A'..'Z')*
;
STMTEND
: SEMICOLON NEWLINE*
| NEWLINE+
;
STRING
: SQUOTED_STRING
| DQUOTED_STRING
;
fragment SQUOTED_STRING
: '\'' (~['])* '\''
;
fragment DQUOTED_STRING
: '"' ( ESC_SEQ| ~('\\'|'"') )* '"'
;
fragment ESC_SEQ
: '\\' ('b'|'t'|'n'|'f'|'r'|'\"'|'\''|'\\')
| UNICODE_ESC
| OCTAL_ESC
;
fragment OCTAL_ESC
: '\\' ('0'..'3') ('0'..'7') ('0'..'7')
| '\\' ('0'..'7') ('0'..'7')
| '\\' ('0'..'7')
;
fragment HEX_DIGIT : '0x' ('0'..'9' | 'a'..'f' | 'A'..'F')+;
fragment UNICODE_ESC : '\\' 'u' HEX_DIGIT HEX_DIGIT HEX_DIGIT HEX_DIGIT;
fragment SEMICOLON : ';';
fragment NEWLINE : '\r'?'\n';