CODEWITHSUNDEEP
javascriptregex
totalnoob
totalnoob

Reputation: 2741

only allow specific patterns with specific characters regex - javascript

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?

Upvotes: 1

Views: 469

Answers (3)

The fourth bird
The fourth bird

Reputation: 163207

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));
});

Upvotes: 1

Nikolas
Nikolas

Reputation: 44368

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*$

Upvotes: 1

mrzasa
mrzasa

Reputation: 23307

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

Upvotes: 0

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