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';