Determine if a string is numeric

# title: Determine if a string is numeric
# source: https://rosettacode.org/wiki/Determine_if_a_string_is_numeric
# category: Rosetta Code
# difficulty: Intro
# summary: Recognize simple signed integer and decimal strings.
# tags: popular, strings, validation, parsing
# vibe: 0.2

def digit?(char)
  char >= "0" && char <= "9"
end

def numeric?(text)
  if text == ""
    return false
  end

  index = 0
  if text.slice(0) == "+" || text.slice(0) == "-"
    index = 1
  end

  if index >= text.length
    return false
  end

  seen_digit = false
  seen_decimal = false

  while index < text.length
    char = text.slice(index)
    if digit?(char)
      seen_digit = true
    elsif char == "." && !seen_decimal
      seen_decimal = true
    else
      return false
    end
    index = index + 1
  end

  seen_digit
end

def run
  [
    { text: "42", numeric: numeric?("42") },
    { text: "-17.5", numeric: numeric?("-17.5") },
    { text: "+.5", numeric: numeric?("+.5") },
    { text: "12a", numeric: numeric?("12a") },
    { text: "--3", numeric: numeric?("--3") }
  ]
end