input: Write a python function to extract the first sequence of digits from each string in a list using regular expressions.
  assert extract_digits(['abc123', '456def', '7890ghi']) == ['123', '456', '7890']
  assert extract_digits(['no1number', 'second2', 'third']) == ['1', '2', '']
  assert extract_digits(['4thstring', 'fifth5', '6six']) == ['4', '5', '6']
output: def extract_digits(strings):
      import re
      result = []
      for s in strings:
          match = re.search(r'\d+', s)
          if match:
              result.append(match.group(0))
          else:
              result.append('')
      return result