How to write a regular expression to validate a variable against 0-100 or an e

Question

I would like to write a regular expression to validate and input field against the following arguments:

1. field is required (cannot be empty)
2. field must not be a negative number
3. field must be a validate decimal number to two decimals (eg. 1 or 1.3 or 1.23)
4. field can be any valid number between 0 and 100 or an 'e'

Answer 1

Hmm does this work for you?

^((100|[0-9]{1,2})(\.[0-9]{1,2})?)|(e)$

Whay environment is this for? Any particular regex standard it must adhere to?

Constraints on numeric values (such as "> 100", or "<= 5.3") can make regexes rather complicated. These types of contraints are better checkedin application logic. Then you can have a simpler (and easier to understand) pattern:

^(([0-9]{1,3})(\.[0-9]{1,2})?)|(e)$

And then extract the capture group for the first 3 digits and validate that separately.

Edit: Ok I think this one should do it (last one because my eyes are getting tired):

^(100(\.0{1,2})?)|([0-9]{1,2})(\.[0-9]{1,2})?|(e)$

Will also allow 100.00 or 100.0

Answer 2

^(?:100|\d{1,2}(?:\.\d{1,2})?|e)$

Answer 3

Regular expressions find great use in checking format, but you're wishing to use it to do a subset of floating point number parsing and bounds checking. Be kind to yourself and the person who will maintain your code after you're gone: check if it's an 'e', else read it into a float and check the bounds.

Answer 4

You can use: ^(100|\d{1,2}(\.\d{1,2})?|e)$

However, it would be simpler and more readable to use your language's float parsing/casting functions.

EDIT: Some variations based on the comments:

Allowing 100.0 and 100.00: ^(100(\.0{1,2})?|\d{1,2}(\.\d{1,2})?|e)$

Disallowing leading zeroes: ^(100(\.0{1,2})?|[1-9]?\d(\.\d{1,2})?|e)$