Ruby: How do I find a letter in a string if only it is at the beginning of said string?

$ sed -r 's/.*,[[:space:]]*([^,[:space:]]+)[[:space:]]*Task.*/\1/' file
192

str = 'teststring'

arr = str.chars
(arr - (arr.difference(arr.uniq))).first
  #= "e"

a = [1,2,3,4,3,2,2,4]
b = [2,3,4,4,4]

a     -      b #=> [1]
a.difference b #=> [1, 3, 2, 2]

arr - arr.difference(arr.uniq)

def nearby_az(str)
  list = Array.new
  pos = -1
  str.each_char do |c|
    pos = pos + 1
    if (c == 'a') then
      list.push(pos+1)
      list.push(pos+2)
      list.push(pos+3)
    end
  end
  list
end

def nearby_az(str)
  list = Array.new
  nstr = str.each_char.to_a
  nstr.each_index do |i|
    if (nstr[i] == 'a') then
      list.push(i+1)
      list.push(i+2)
      list.push(i+3)
    end
  end
  list
end

puts nearby_az("asdfgaqwer")

def nearby_az(string)
  list = []
  for i in 0..(string.length)
    if string[i] == 'a'
      list.push(i+1)
      list.push(i+2)
      list.push(i+3)
    end
  end
  list
end

$string = 'Aorem ipsum adolor sit amet, consectetur dipiscing aelit.';
echo "<br>".preg_replace('/(\ba\w+\b|\bA\w+\b)/', '', $string);

ipsum sit , consectetur dipiscing .