input: Write a python function to match a pattern in a string using regular expressions.
  assert match_pattern("abc:123") == True
  assert match_pattern("abc123") == False
  assert match_pattern("abc:") == False
  assert match_pattern("abc:123:456") == False
output: import re
  
  def match_pattern(s):
      pattern = r'^[a-z]+:[0-9]+$'
      return bool(re.match(pattern, s))