function checkEmail(str){
    var emailCheck = /[^a-z][0-9]/;
    if(emailCheck.test(str)){
      console.log("**Its Valid Mail");
    }
    else{
      console.log("Its not Valid");
    }
}
var x = '122a';
checkEmail(x);

I have been learning Regular Expressions. From the above code what I understand, x should not contain small a-z and it must contain number, as you can see 122a contains number as well as small letter a, I think it should be invalid but yet I get it as valid. Can anyone please explain where I am thinking wrong.

function checkEmail(str){
    var emailCheck = /[^a-z][0-9]/;
    if(emailCheck.test(str)){
      console.log("**Its Valid Mail");
    }
    else{
      console.log("Its not Valid");
    }
}
var x = '122a';
checkEmail(x);

I have been learning Regular Expressions. From the above code what I understand, x should not contain small a-z and it must contain number, as you can see 122a contains number as well as small letter a, I think it should be invalid but yet I get it as valid. Can anyone please explain where I am thinking wrong.

• javascript
• regex

3 Answers 3

The problem is, you are not checking the start and end of the string.

Specify the Start and End of your Regex:

/^[^a-z][0-9]+$/

The caret ^ and dollar $ characters have special meaning in a regexp. They are called anchors. The caret ^ matches at the beginning of the text, and the dollar $ – in the end.

function checkEmail(str){
  var emailCheck = /^[^a-z][0-9]+$/;
  if(emailCheck.test(str)){
    console.log("**Its Valid Mail");
  }
  else{
    console.log("Its not Valid");
  }
}
var x = '122a';
checkEmail(x);

For more: Start of String and End of String Anchors in Regular Expression

Try /^[0-9]+$/g.test(str).

> var s = '122a'
undefined
>
> /^[0-9]+$/g.test(s)
false
>
> var s = '122'
undefined
>
> /^[0-9]+$/g.test(s)
true
>

You regex [^a-z][0-9] matches 2 characters. The first would match not a-z using a negated character class and the second will match a digit. That will match 12 in 122a

The method test searches for a match in a string, finds a match 12 and will return true.

You could use anchors to assert the start ^ and the end $ of the string to match 2 characters. In that case, 122a will not match because that are 4 characters.

If you want to check if the string does not contain a lowercase character and does contain a digit you could use a lookahead to assert that what followes is not [a-z] and that what follows is digit [0-9].

^(?!.*[a-z])(?=.*[0-9]).*$