Haskell: check if string is valid number

Question

How do I check for a decimal point when checking a string is a valid number?

What I am thinking is that I use something like the following, but add code to check for the decimal point!

isNumber :: String -> Bool
isNumber xs = all isDigit xs || add extra code here

Where a valid number is defined in EBNF as:

number -> .digit+ | digit+ [ .digit*]

For example, .5, 1.5, 1, 1. are all valid numbers. + signifies one or more occurrences, and * denotes zero or more.

Answer 1

A simple approach involves using readMaybe for converting a string into a number,

import Text.Read

and so for checking whether it is a Double,

readMaybe "123" :: Maybe Double
Just 123.0

readMaybe "12a3" :: Maybe Double
Nothing

The latter returns Nothing, the string is not a valid number. In a similar fashion, if we assume it is an Int,

readMaybe "12.3" :: Maybe Int
Nothing