only allow specific patterns with specific characters regex - javascript

Question

I'm writing a method to check if input matches either

>=n, >n, <n, <=n or n..n.

Regex has always been something that's been confusing. I feel this would be a good example to understand it better.

so far I'm just checking if it has those characters

const text = '>2';
const regex = /^[<>=.0-9]|g/i;
console.log(regex.test(text));

how do I create a regex that'll only allow those specific patterns / quantifiers? eg. >5 is valid but 5> is not.

what's the terminology behind these types of things?

You can use: /^[<>]=?\d+\.?\d*$/

anubhava
– anubhava

2018-11-28 21:18:23 +00:00
Commented Nov 28, 2018 at 21:18 — anubhava
– anubhava, Commented Nov 28, 2018 at 21:18

Nikolas · Accepted Answer · 2018-11-28 21:40:44Z

1

You have to mind the order and be more exact:

^(?:>|<|>=|<=)(?:[1-9]\d*|0)$

(?:>|<|>=|<=) is the set of valid operators
(?:[1-9][0-9]*|0) is the number without leading zero

So the full regex variable would be initialized as:

regex = /^(?:>|<|>=|<=)(?:[1-9]\d*|0)$|g/i;

You have included to match n..n as an alternative. Here you go:

^((>|<|>=|<=)|([1-9]\d*|0)\.\.)([1-9]\d*|0)$

\. matches the . dot literally and must be escaped

Test it:

regex = /^((>|<|>=|<=)|([1-9]\d*|0)\.\.)([1-9]\d*|0)$|g/i;

array = new Array();

array[0] = ">2";    // valid
array[1] = ">0";    // valid
array[2] = "2..3"   // valid
array[3] = "=2";    // invalid
array[4] = ">01";   // invalid
array[5] = "2>";    // invalid

array.forEach(item => console.log(regex.test(item)));

If you don't mind the leading zero, simply use the:

^((>|<|>=|<=)|\d*\.\.)\d*$

edited Nov 28, 2018 at 21:40

answered Nov 28, 2018 at 21:22

Nikolas

45k19 gold badges132 silver badges201 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

totalnoob Over a year ago

how about for n..n?

The fourth bird · Accepted Answer · 2018-11-28 21:46:08Z

1

You could use an alternation to match either > or < followed by an optional = or match a digit followed by 2 dots.

After the alternation you could match a digit.

^(?:[<>]=?|\d\.\.)\d$

That will match:

^ Start of the string
(?: Non capturing group
- [<>]=? Match < or > or optional =
- | Or
- \d\.\. Match a digit and 2 dots
) Close non capturing group
\d Match a digit
$ Assert the end of the string

Regex demo

const strings = [
  ">=3",
  ">3",
  "<2",
  "<=3",
  "5..5",
  "5>"
];
let pattern = /^(?:[<>]=?|\d\.\.)\d$/;
strings.forEach(s => {
  console.log(pattern.test(s));
});

edited Nov 28, 2018 at 21:46

answered Nov 28, 2018 at 21:18

The fourth bird

165k16 gold badges61 silver badges75 bronze badges

Comments

mrzasa · Accepted Answer · 2018-11-28 21:17:41Z

0

You can probably use ^[<>]=?\d+$

^ match from the beginning of the string
[<>] mandatory greater or less mark
=? optional equal mark
\d+ any number of digits
$ match till the end of the string

Demo

answered Nov 28, 2018 at 21:17

mrzasa

23.4k11 gold badges60 silver badges96 bronze badges

Collectives™ on Stack Overflow

only allow specific patterns with specific characters regex - javascript

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related