Can't figure out why the pattern will match a period but not a square bracket.

For example, it will match "See Jack run." but not "See [Jack] run.". Just ignores the brackets.

Any ideas? Thx!

var title = note_title.value;

// validate periods and brackets	
var patt = '\\.\\[\\]';
var mod = 'g';
var dupe_patt = new RegExp(patt,mod);
  console.log('no periods or brackets allowed in title');
  return false;