All characters in a string must match regex
You need * or + quantifier after your character class and anchors:
/^[xX<>=|0-9&()\s.]*$/.test(expression)
^ ^^
Now, it will match
-
^- start of string -
[xX<>=|0-9&\s().]*- zero or more (if you use+, one or more) of the chars defined in the char class -
$- end of string.
Short demo:
console.log(/^[xX<>=|0-9&\s().]*$/.test("a.1"));
console.log(/^[xX<>=|0-9&\s().]*$/.test("(x>1) && (x<2)"));Note I added \s to match whitespaces, too.