Having trouble understanding regular expressions in JavaScript

I wrote a regex to weed out bad calculator inputs (disallow consecutive decimal points, no decimal followed by an operation, i.e., +.1, etc.). So, the regex looks like this:

const regex = /\.[+-/x]|\.[0-9]+\.| \.+|[+-/x]\./  

And it works - anything that matches this is weeded out. My problem is, I don't know why it works. Also, this doesn't work, for some reason, if I delete the space between the second-to-last "OR" operator and the next escape character. Can someone explain this? Also, is there a simpler, equivalent, regular expression I could use? This expression results in the following inputs/outputs:

!regex.test('3.33')   // true  !regex.test('3.33.3')  // false  !regex.test('0.333')   // true  !regex.test('+.')     // false  !regex.test('.-')    // false  

https://stackoverflow.com/questions/66610238/having-trouble-understanding-regular-expressions-in-javascript March 13, 2021 at 12:49PM