正则表达式以字母或下划线 (_) 开头

Question

我正在尝试检查以下划线 (_) 或字母开头的字符串，并且只能包含字母、数字、连字符、下划线或句点。 字符串的长度也可以是 1。

预期的有效字符串：

1. _姓名
2. _name.First
3. 名字优先
4. 姓名
5. 姓名.名字
6. 名字优先
7. 一个
8. b

我尝试使用下面给定的正则表达式，但不适用于单个字母。

^[a-zA-Z0-9_][a-zA-Z0-9_|/.|/-]{1,20}[a-zA-Z0-9]$

Answer 1

使用

^[a-zA-Z0-9_](?:[a-zA-Z0-9_.-]{0,20}[a-zA-Z0-9])?$

见proof。

说明

                         EXPLANATION
--------------------------------------------------------------------------------
  ^                        the beginning of the string
--------------------------------------------------------------------------------
  [a-zA-Z0-9_]             any character of: 'a' to 'z', 'A' to 'Z',
                           '0' to '9', '_'
--------------------------------------------------------------------------------
  (?:                      group, but do not capture (optional
                           (matching the most amount possible)):
--------------------------------------------------------------------------------
    [a-zA-Z0-9_.-             any character of: 'a' to 'z', 'A' to
    ]{0,20}                  'Z', '0' to '9', '_', '.', '-' (between
                             0 and 20 times (matching the most amount
                             possible))
--------------------------------------------------------------------------------
    [a-zA-Z0-9]              any character of: 'a' to 'z', 'A' to
                             'Z', '0' to '9'
--------------------------------------------------------------------------------
  )?                       end of grouping
--------------------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string