regex for number between numbers

Question

I'm in need of a regex, which takes a minimum and a maximum number to determine valid input, And I want the maximum and minimum to be dynamic.

I have been trying to get this done using this link https://stackoverflow.com/a/13473595/1866676 But couldn't get it to work. Can someone please let me know how to do this.

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Let's say I want to make a html5 input box, and I Want it to only receive numbers from 100 to 1999

What would a regex for this like this look like?

Answer 1

First off, while it is possible to do this, I think if there is a simpler way to choose a number range such as <input type="number" min="1" max="100">, that way would be preferred.

Having said that, here's how the kind of regex you requested works:

ones: ^[0-9]$  // just set the numbers -- matches 0 to 9
tens: ^[1-3]?[0-9]$ //set max tens and max ones -- matches 0 to 39
tens where max does not end in 9 ^[1-2]?[0-9]$|^[3][0-4]$ // 0 to 34
only tens: ^[1][5-9]$|^[2-3][0-9]$|^[4][0-5]$ // 15 to 45

Here, lets pick an arbitrary number 1234 to 2345

^[1][2][3][4-9]$|
^[1][2][4-9][0-9]$|
^[1][3-9][0-9][0-9]$|
^[2][0-2][0-9][0-9]$|
^[2][3][0-3][0-9]$|
^[2][3][4][0-5]$

https://regex101.com/r/pP8rQ7/4

Basically the ending of the middle series always needs to be a straight range that can reach 9 unless we are dealing with the ones place, and if it cant, you have to build it upwards toward the middle each time we have a value that can't start in 0 and then once we reach a value that cant end in 9 break early and set it in the next condition.

Notice the pattern, as each place solidifies. Also keep in mind that when dealing with going from lower to higher places, optional operators ? should be used.

Its a bit complex, but its nowhere near impossible to design a custom range with a bit of thought.

If you are more specific, we can craft an exact example, but this is generally how it is done:beginning-range|middle-range|end-range You should only need beginning or end-ranges in certain cases like if the min or max does not end in 9. the ? means that the range that comes after it is optional. (so for example in the first case it lets us have both single and double numbers.

so for 100 - 1999 it's quite simple actually because you have lots of 9's and 0's

/^[1-9][0-9][0-9]$|^[1][0-9][0-9][0-9]$/

https://regex101.com/r/pP8rQ7/1

Note: Single values don't need ranges [n] I just added them for readability.

Edit: There used to be a regex range generator at: http://gamon.webfactional.com/regexnumericrangegenerator/. It appears to be offline now.

Answer 2

Essentially, you can't.

For every numeric range, there exists a regex that will match numbers in that range, therefore it is possible to write code that can generate a such regex. But such a regex is not a simple reformatting of the range ends.

However, such code would require colossal effort and complexity to write compared to code that simply checked the number using numeric methods.