正则表达式不是适合该工作的工具.手动扫描字符串.

Regex is not the right tool for the job. Scan a string manually.

伪代码:

depth = 0
for character in some_string:
    depth += character == '('
    depth -= character == ')'
    if depth < 0:
       break

if depth != 0:
   print "unmatched parentheses"