Basic logic implemented

.gitignore

+*.sublime-*
\ No newline at end of file

PolicyTest.php

+<?php
+// declare(strict_types=1);
+
+use PHPUnit\Framework\TestCase;
+include 'sanitize.php';
+
+final class PolicyTest extends TestCase
+{
+    public function testPolicy_usernames(): void
+    {
+    	// Remove all characters after the first "@"
+        $this->assertEquals(
+        	sanitize_username('foo@example.com'),
+        	'foo'
+        );
+
+		// Remove all remaining non-alphanumeric characters        
+        $this->assertEquals(
+        	sanitize_username('valid+email@example.com'),
+        	'validemail'
+        );
+        
+        $this->assertEquals(
+        	sanitize_username('valid.email@example.com'),
+        	'validemail'
+        );
+        
+        $this->assertEquals(
+        	sanitize_username('valid-email@example.com'),
+        	'validemail'
+        );
+        
+        $this->assertEquals(
+        	sanitize_username(')(*&l^%!o#$^l%1#3^3#7$'),
+        	'lol1337'
+        );
+
+        // If username starts with a numeral, prepend an "x"
+        $this->assertEquals(
+        	sanitize_username('029822c@example.com'),
+        	'x029822c'
+        );
+        $this->assertEquals(
+        	sanitize_username('123456789@example.com'),
+        	'x123456789'
+        );
+
+        // Set all letters to lowercase
+        $this->assertEquals(
+        	sanitize_username('exAmpleRWEFS@example.com'),
+        	'examplerwefs'
+        );
+    }
+}

phpunit.xml

+<?xml version="1.0" encoding="UTF-8"?>
+
+<phpunit backupGlobals="false"
+         backupStaticAttributes="false"
+         colors="true"
+         convertErrorsToExceptions="true"
+         convertNoticesToExceptions="true"
+         convertWarningsToExceptions="true"
+         processIsolation="false"
+         stopOnFailure="false"
+>
+    <testsuites>
+        <testsuite name="Purism username plugin - Test Suite">
+            <directory>.</directory>
+        </testsuite>
+    </testsuites>
+</phpunit>
+

purism-username-policy.php

+<?php
+/*
+ * @package Purism-Username-Policy
+ * @version 0.1
+Plugin Name: Purism's usernames policy
+Plugin URI: https://source.puri.sm/liberty/internal/issues/32
+Description: Wordpress creates usernames from email addresses. These could be invalid Purist usernames, for example p.Sherman. These should be autocorrected to follow Purist rules, for example psherman.
+Author: Copyright 2018 Purism SPC
+Version: 0.1
+Author URI: http://www.puri.sm/
+*/
+
+function purism_pre_user_login_filter($sanitized_user_login){
+    // do your sanitization with $sanitized_user_login
+    return $sanitized_user_login;
+}
+add_filter('pre_user_login', 'purism_pre_user_login_filter', 10, 3);
+
+

sanitize.php

+<?php
+/**
+ * Wordpress creates usernames from email addresses. These could be invalid Purist
+ * usernames, for example p.Sherman. These should be autocorrected to follow Purist
+ * rules, for example psherman.
+ * 
+ * - Remove all characters after the first "@"
+ * - Remove all remaining non-alphanumeric characters
+ * - If username starts with a numeral, prepend an "x"
+ * - Set all letters to lowercase
+ * - Other Wordpress logic should apply (e.g. avoid duplicates by adding a number)
+ */
+function sanitize_username($login_name)
+{
+    // Remove all characters after the first "@"
+    $sane_name = explode('@', $login_name)[0];
+
+    // Remove all remaining non-alphanumeric characters
+    $sane_name = preg_replace("/[^a-zA-Z0-9]+/", "", $sane_name);
+
+    // If username starts with a numeral, prepend an "x"
+    if (preg_match('/^\d/', $sane_name) === 1) {
+        $sane_name = 'x' . $sane_name;
+    }
+
+    // Set all letters to lowercase
+    $sane_name = strtolower($sane_name);
+
+    return $sane_name;
+}