Dynamic value for max length in a regex

问题

i need to validate differents input value lengths.

Inputs value can have a max length of 6 / 15 / 25 characters. Now i was asking to my self if it's a good practice use only one dynamic regex to validate differents max lengths, instead of copy paste the same regex.

During my research i found that i've to use the the const regex = new RegExp()

the problem is that i tried

const lengthValidation = () => {
  const maxLength = 4;
  const inputValue = 'ciao';
  const regex = new RegExp(`/^.{6,${maxLength}}$/`);
  const isValid = regex.test(inputValue);
  return console.log('regexTest', isValid);
};

lengthValidation();

but it's invalid.

where is my mistake ?

回答1:

As said in the comments, you could simply use the + operator as in

const lengthValidation = () => {
  const maxLength = 4;
  const inputValue = 'ciao';

  let expression = '^.{' + maxLength + ',6}$';
  const regex = new RegExp(expression);

  const isValid = regex.test(inputValue);
  return console.log('regexTest', isValid);
};
lengthValidation();

Be aware though that e.g. .{6,4} will lead to an error as the engine requires the quantifiers to be in the correct order. You might build in a check before otherwise the expression will fail.
Additionally, for a simple length check, a regular expression might be a bit of an overkill.

回答2:

You should remove the slashes from the regex string. Slashes are only needed to create a Regex when you are using the var regex = /expr/; notation.

Also, you need to use a lower bound of e.g. 1 character, not 6:

const lengthValidation = () => {
  const maxLength = 4;
  const inputValue = 'ciao';
  const regex = new RegExp(`^.{1,${maxLength}}$`);
  const isValid = regex.test(inputValue);
  console.log(regex);
  console.log('regexTest', isValid);
};

lengthValidation();

来源：https://stackoverflow.com/questions/59875470/dynamic-value-for-max-length-in-a-regex

标签

javascript

regex

validation