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Check Whether a String Is a Signed Decimal Number Without Exponent Notation

LinkedInSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenCoding & Algorithms

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Write a function that decides whether a string is a valid plain decimal number with an optional sign and fractional part, rejecting exponents, stray symbols, and repeated decimal points. It tests turning a small grammar into careful character-by-character validation and covering edge cases such as empty strings and bare signs.

Implement a function that decides whether a string is a valid number in plain decimal notation: an optional sign, digits, and an optional fractional part. Exponent notation and any other symbols are not allowed.

Function Signature

is_number(to_test: str) -> bool

Rules

A string is valid exactly when the whole string matches this form, with nothing before or after it:

  1. An optional single sign character, + or -.
  2. One or more decimal digits 0 through 9.
  3. Optionally, a single . followed by one or more decimal digits.

Consequences of the rules:

  • "234", "-455", and "23.23" are valid.
  • "1e2" is invalid because exponent notation is not supported, and "12^2" is invalid because ^ is not allowed.
  • "2.2.4" is invalid because at most one decimal point may appear.
  • Leading zeros are allowed, so "007" and "-0" are valid.
  • "", "-", ".5", "5.", and any string containing whitespace are invalid.

Accepting +, allowing leading zeros, and requiring digits on both sides of a decimal point are conventions of this exercise.

Constraints

  • 0 <= len(to_test) <= 100.
  • to_test contains only printable ASCII characters, including spaces.

Examples

Input: to_test = "-455"

Output: True

Input: to_test = "23.23"

Output: True

Input: to_test = "2.2.4"

Output: False

The second decimal point makes the string invalid.