正则表达式模式匹配难度

Question

I'm having issues with understanding pattern matching. I've figured it out to a point but I'm having a lot of trouble with this specific pattern I need to match.

Requirements: Check for the format of the user name. It should start with a letter followed by letters, numbers, and periods. A user name should be at least 6 characters long but no more than 12.

This is what I have so far:

var user = document.getElementById('username').value;
var pos = user.search(/^[A-z](?=.*?[A-z])(?=.*?[0-9])\./);

Answer 1

Try this pattern

var pattern = /^[A-Za-z][A-Za-z.0-9]{5,11}$/


^ matches beginning of file. 
[A-Za-z] = matches first letter
[A-Za-z.0-9] = Matches a-z both A-Z and a-z the . (dot) character and numbers 0-9
{5,11} = tells that the last character "group" should have between 5 or 11 occurrences. witch makes the total string between 6 and 12 characters long. 
$ = matches end of string

Hope this helps!

Edit:

Javascript to do the match

var pattern = /^[A-Za-z][A-Za-z.0-9]{5,11}$/

//assuming the var username contains the username
if(username.match(pattern)){
     alert("Valid pattern");
}
else{
     alert("Invalid pattern")
}

Answer 2

It should start with a letter followed by letters, numbers, and periods. A user name should be at least 6 characters long but no more than 12.

So you want to allow a string that starts with a letter and has at least one dot or number and is between 5 and 12 characters.

In that case, use the following:

^[A-Za-z](?=.*\d.*)(?=.*[.].*)[A-Za-z\d.]{5,11}$

• ^ match start.
• [A-Za-z] a letter.
• (?=.*\\d.*) positive lookahead - the following string contains at least 1 digit.
• (?=.*[.].*) positive lookahead - the following string contains at least 1 dot.
• [A-Za-z\\d.]{5,11} the rest of the string contains between 5 and 11 letters, digits and dots (the total string between 6 and 12).
• $ match end.

Note: -

• ?= the forward lookup does not affect the match position, so the match will continue at the second character.
• ?= an AND is implied between the forward lookups, so both need to be satisfied.

Answer 3

^[a-zA-Z][a-zA-Z.]{5,11}$

这个应该适合你，因为你没有任何关于任何角色最小出现的约束。