允许特定用户名的PHP正则表达式[关闭]

Question

This is my first question here. Currently I'm developing a PHP forum script and I want to let people register a username abiding by my rules.

The rules are simple:

• First character must be a letter
• They can also use numbers but it isn't the first
• They can use fullstop/underscore once but not at the end of a username

Please help me by putting down a function. I read some tutorial but don't understand regular expressions well. And for this reason I'm stuck. Thanks in advance for your answer.

Added after 20 min:

Thanks for answering, I have one more thing to ask, Can I detarmine dumb usernames?? E.G. aaa123, ab-cd,uuuu...Than How?

Answer 1

Take the parts you describe:

• First character must be a letter

It starts ^[A-Za-z]

• They can also use numbers but it isn't the first

[A-Za-z0-9]* in the middle

• They can use fullstop/underscore once but not at the end of a username

Optionally, have an underscore followed by at least one of the other valid characters ([._][A-Za-z]+)?

Put it all together and you have: /^[A-Za-z][A-Za-z0-9]*([._][A-Za-z0-9]+)?$/. Using the case '/i' insensitive flag lets you drop the 'A-Z': /^[a-z][a-z0-9]*([._][a-z0-9]+)?$/i.

Regular expressions are equivalent to deterministic finite automatons (DFA). Studying them can help get a grasp on regular expressions. In particular, transitioning between states is directly applicable to your three points. The (reduced) state diagram for this language should be quite easy to understand:

The DFA:

• starts in state 0
• transitions to state 1 if a letter is encountered,
• stays in state 1 as long as letters and digits are encountered,
• switches to state 2 if a period or underscore are encountered,
• switches to state 3 if a letters and digit is encountered,
• stays in state 3 as long as letters and digits are encountered,
• accepts the string if it ends in state 1 or 3

I call it "reduced" because there's a fifth, non-accepting state and edges leading to it that aren't shown. Basically, if a character is encountered other than one listed, the DFA transitions to the fifth, non-accepting state and stays there.