javascript regex check character before match but don't include it in result

Question

I'm trying to implement the two following rules in a single regex:

If a number is preceeded by :

• a word character or a @ character ; then do not match anything
• anything else ; then match the number but do not include the preceeding character in the result

I tried: [^@\\d,\\w]\\d+ and (?:[^@\\d,\\w])\\d+

Which solve the first rule, but fail to solve the second one, as it includes the operator in the result.

I understand why it doesn't work as intended ; the [^@\\d\\w] part explicitly says to not match numbers preceeded by a @ or a word character, so it implicitly says to include anything else in the result. Problem is I still have no idea how to solve this.

Is there any way to implement the two rules in a single regex ?

Input string:

@121 //do not match
+39  //match but don't include the + sign in result
s21  //do not match
89   //match
(98  //match but don't include the ( in result
/4   //match but don't include the / operator in result

Expected result:

39 //operator removed
89  
98 //( removed
4  //operator removed

Answer 1

Capture the result you're seeking as the snippet below suggests.

See regex in use here

^[^@\w]?(\d+)

• ^ Assert position at the start of the line
• [^@\\w]? Optionally match any character except @ or a word character
• (\\d+) Capture one or more digits into capture group 1

 var a = ["@121", "+39", "s21", "89", "(98", "/4"] var r = /^[^@\\w]?(\\d+)/ a.forEach(function(s){ var m = s.match(r) if(m != null) console.log(m[1]) }) 

Answer 2

There's a finished proposal for negative lookbehind which I think it's what you are looking for:

 let arr = ['@121', //do not match '+39', //match but don't include the + sign in result 's21', //do not match '89', //match '(98', //match but don't include the ( in result '/4' //match but don't include the / operator in result ]; console.log(arr.map(v => v.match(/(?<![@\\w])\\d+/))); 

It's a bleeding edge feature however (works on chromium 62+ i think).