How to generate username of specific pattern in drupal during registration - module

Question

I want to generate a usernames st + first two initials of their first name + their last name.
There are few modules to create username of specific pattern but none of them can accomplish my requirement. I am not Drupal module expert.

Answer 1

What you can do is to install the automatic username module:
Navigate to the configuration page of this module: admin/config/people/accounts/patterns
Then, in the "Other settings" section you can check the option "Evaluate PHP in pattern."
This allows you to execute PHP in the "Pattern for username" field. Two things to consider:
PHP is executed after the token replacement, this allows you to actually manipulate the strings.
You need to echo the final result.
Make sure that the php is wrapped with <?php ?> tags. Also, if you make a syntax error, it will only be seen upon registration (Or whenever the script is executed.)
Now let's say you configured your users to have a first_name field and a last_name field (with those specific field names)
Your PHP could look something like this:
<?php
$initials = substr("[user:field-first-name]", 0, 2);
echo 'str' . $initials . "[user:field-last-name]";
?>
First, Note that the token are between double quotes to make sure that what is outputted by the token is considered a string by PHP
Second, the php function substr is used to get the 2 first letters of the first name.
Now, things I don't know
I have no idea what happens if two users happen to have the same resulting username
I don't know what happen if a user enters a double quote in his name, might be dangerous, you might want to escape it with a str_replace or something like that