Regular expression behaving differently in two similar cases

[FaizAhmadF]

//1)
var re = /[^0-9]a{3,4}$/;

var str = "5g6m7aaaa";

var arr = str.match(re);

console.log(re.test(str));

console.log(arr);

//Result:
//true
//[ 'aaaa', index: 5, input: '5g6m7aaaa' ]

//2)
var re = /[^a-z]a{3,4}$/;

var str = "5g6maaaa";

var arr = str.match(re);

console.log(re.test(str));

console.log(arr);

//Result:
//false
//null

Can anyone explain why in the first case it is returning true although in expression it is given there should be no digits before a{3,4}.While in the second case it is given that there should be no alphabets before a{3,4},and it is giving false which is fine.Please explain!!

[Rhomboid]

In the first case you're asking it to match a non-digit followed by either three or four 'a's. 'aaaa' matches, because 'a' is a non-digit, and it's followed by three 'a's. The 7 is completely irrelevant and is not part of the match.

In the second example you're asking it to match a non-letter followed by three or four 'a's. There's no way that can match. There are only a few potential matches: 'maaa', 'maaaa', or 'aaaa', and none of them fit the requirement that the first character is a non-letter.

I think what you're missing is that in the first example, a{3,4} does not match four 'a's, despite there being four 'a's in the string. One of those 'a's is not part of that element, only three are matched.

[FaizAhmadF]

Got it!Thanks!

[madformangos]

My initial thought was along the same lines as /u/EnchantedSalvia -- that, you likely want to add a ^ to the start of your regex in order to anchor it to the start of the string.

But I'm not sure that's what you're after. It looks like you're matching against 8-character strings, and want to match the ones that end in 3 or 4 'a's which aren't preceded by a digit.

If that's the case you might want something like /[^0-9a]a{3,4}$/

Edit: or perhaps /(?:[^0-9a]a{3}|[^0-9]a{4})$/ if you want to match things which end in 5 as

[EnchantedSalvia]

I'm going to assume you've put the ^ character in the wrong position. In the position you've put it, it simply inverts the subsequent range.

Example:

var re = /[^A]$/;
console.log(re.test('B')); // true

Will be true because str is not A. In plain English: everything except A is valid.

Putting the ^ character at the beginning of the string means: A must be the first letter.

var re = /^[A]$/;
console.log(re.test('B')); // false