正则表达式允许空字符串

Question

I'm trying to apply a regex in which a number can be formatted to two decimal places, I've been able to achieve this. But I am unable to delete the first character in the string because my test will fail if there is no match. How can I modify this regex to support the deletion of the entire value?

/^[0-9]+(\\.([0-9]{1,2})?)?$/

https://codesandbox.io/s/k2qlq0w68r

Answer 1

For the specific thing that you're doing (input validation), you can probably just eschew your regex altogether and just use <input type="number"> with a step :

<input type="number" step="0.01">

You can check the validity property to check if anything is incorrect. I believe something like document.getElementById("id").checkValidity() will give you what you want.

See also: Validating forms using JavaScript and Form Validation Part 2: The Constraint Validation API (JavaScript) .

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If you still wish to use the regex, it's probably easier to separate the only-decimal and part non-decimal cases:

/^(?:\d+(?:\.\d{0,2})?|\.\d{1,2})$/

This matches the following numbers: 100 , 12.5 , 10. , 8.51 , .67 , .0

But rejects the following: .032 , . , [empty string]

I would recommend that you run the validation after the user types in the number, not as they are currently typing it in. Otherwise, it will be impossible for them to type .01 , as your regex will reject them when pressing . .

Answer 2

You can you this regex if you want to make decimal before dot optional

^([0-9])?(?(1)(.([0-9]{1,2})?)?|(.[0-9]{1,2}))$

Is this what you want?

Answer 3

If you want a regex that also accepts the empty string:

/^([0-9]+(\\.([0-9]{1,2})?)?|)$/

But I would suggest to you to not assume that the input is valid while the user puts it into your input but instead validate the value after the user is done editing (eg by using a loose focus event).