用于过滤不需要的用户名的正则表达式

Question

在我正在开发的网站上，我要求用户名不能以 _ 开头，

因此不应允许这些：

• SU_Coolguy
• MR_Nobody
• my_Pony

但这些应该没问题：

• __SU_Coolguy
• MRS_Nobody
• YourPony

在我使用的框架中，我只能验证匹配的正则表达式，而不能验证不匹配的情况。到目前为止我已经想出了这个：

"/^([^A-Za-z0-9]{2}\_|[A-Za-z0-9]{3,27})/"

这适用于大多数项目，但在“__SU_Coolguy”上失败。

对此正则表达式的任何帮助将不胜感激。 :)

Answer 1

这使用 负向预测：

^(?![A-Za- z]{2}_)[A-Za-z0-9_]{3,27}$

让我们分解一下：

Assert position at the beginning of a line (at beginning of the string or after a line break character) «^»
Assert that it is impossible to match the regex below starting at this position (negative lookahead) «(?![A-Za-z]{2}_)»
   Match a single character present in the list below «[A-Za-z]{2}»
      Exactly 2 times «{2}»
      A character in the range between “A” and “Z” «A-Z»
      A character in the range between “a” and “z” «a-z»
   Match the character “_” literally «_»
Match a single character present in the list below «[A-Za-z0-9_]{3,27}»
   Between 3 and 27 times, as many times as possible, giving back as needed (greedy) «{3,27}»
   A character in the range between “A” and “Z” «A-Z»
   A character in the range between “a” and “z” «a-z»
   A character in the range between “0” and “9” «0-9»
   The character “_” «_»
Assert position at the end of a line (at the end of the string or before a line break character) «$»

Answer 2

您的正则表达式将为 /^(?![a-zA-Z0-9]{2}_)/。它的意思是“以不是{两个字母数字字符和一个下划线}开头”。

Answer 3

只需输入一个否定断言，如下所示：

/^([^A-Za-z0-9]{2}(?!\_)|[A-Za-z0-9]{3,27})/
                   ^^--Notice the assertion.

这是一个完整的测试用例：

<?php
$names = array('SU_Coolguy','MR_Nobody','my_Pony','__SU_Coolguy','MRS_Nobody','YourPony');

foreach($names as $name){
        echo "$name => ";
        if(preg_match('/^([^A-Za-z0-9]{2}(?!\_)|[A-Za-z0-9]{3,27})/',$name)) {
                echo 'ok';
        }else{
                echo 'fail';
        }
        echo "\n";
}
?>

输出如下：

SU_Coolguy => fail
MR_Nobody => fail
my_Pony => fail
__SU_Coolguy => ok
MRS_Nobody => ok
YourPony => ok

Answer 4

在这种情况下，一个简单的方法是将其分成不同的情况：用户名要么以非字母开头，要么以字母和非字母开头，要么以两个字母和非下划线开头，大致如下：

/^([^A-Za-z]|[A-Za-z][^A-Za-z]|[A-Za-z][A-Za-z][^_])/