fix

Author: linzhijun

g4/math.g4

@@ -354,9 +354,9 @@ OPOR: '||';
 
 NUM: [0-9]+ ('.' [0-9]+)? ('E' [+-]? [0-9]+)?;
 STRING:
-	'\'' (~'\'' | '\\\'')* '\''
-	| '"' ( ~'"' | '\\"')* '"'
-	| '`' ( ~'`' | '\\`')* '`';
+	'\'' (~['\\] | '\\' .)* '\''
+	| '"' (~["\\] | '\\' .)* '"'
+	| '`' (~[`\\] | '\\' .)* '`';
 
 
 UNIT: