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}$)(?!.*[_.]{2})[^_.].*[^_.]$

CodePudding user response：

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] )$

CodePudding user response：

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