如何为仅以字母开头的用户名制作正则表达式？

Question

I'm trying to create a regular expression that will allow only specific usernames

• Only starts with alphabet.
• Only ends with either number or an alphabet.
• Only allowed - if contains alphabets only or only one dot with alphabets, only one underscore with alphabets.
• Cannot starts with dot or underscore
• Length of username should be only in 4 to 10 char

this is what i am doing - ^(?=[a-zA-Z0-9._]{4,10}$)(?..*[_.]{2})[^_.].*[^_.]$

Answer 1

This should work for you:

^(?=[a-zA-Z0-9._]{4,10})[a-zA-Z](?:[a-zA-Z]+_[a-zA-Z0-9]+|[a-zA-Z]+\.[a-zA-Z0-9]+|[a-zA-Z0-9]+)$

Answer 2

If the only problem currently is that it should not start with numbers, you could use one of these patterns that start the match with a char [A-Za-z] and ends the match with [a-zA-Z0-9]

In the middle there can only be 1 occurrence of either . or _

^(?=[a-zA-Z0-9._]{4,10}$)(?!.*[_.]{2})[A-Za-z]+(?:[_.][a-zA-Z0-9]+)?$

Regex demo

Or another variation:

^[a-zA-Z](?=[a-zA-Z0-9._]{3,9}$)[a-zA-Z0-9]*(?:[._][a-zA-Z0-9]*)?[a-zA-Z0-9]$

Regex demo