Exercise number

Write a function that given a string it return true if the string is a number. As there might be several definitions of what is the number create several solutions one for each definition:

  • Non negative integer.
  • Integer. (Will you also allow + in front of the number or only - ?
  • Real number. (Do you allow .3 ? What about 2. ?
  • In scientific notation. (something like this: 2.123e4 )

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