user can set custom_domain.auto_create_regex

app/dashboard/templates/dashboard/domain_detail/info.html

@@ -12,23 +12,13 @@
   <div class="small-text">Created {{ custom_domain.created_at | dt }}. {{ nb_alias }} aliases</div>
 
   <hr>
-  <h3 class="mb-1">Catch All</h3>
-  <div class="">
-    Create aliases <b>on the fly</b>.
+  <h3 class="mb-1">Auto create/on the fly alias </h3>
 
-    Simply use <em>anything@{{ custom_domain.domain }}</em>
-    next time you need an alias: it'll be <b>automatically</b>
-    created the first time it receives an email. <br>
-    The new alias will belong to
-    {% for mailbox in custom_domain.mailboxes %}
-      <b>{{ mailbox.email }}</b>
-      {% if not loop.last %},{% endif %}
-    {% endfor %}
-  </div>
 
   <div>
     <form method="post">
       <input type="hidden" name="form-name" value="switch-catch-all">
+
       <label class="custom-switch cursor mt-2 pl-0"
              data-toggle="tooltip"
           {% if custom_domain.catch_all %}
@@ -41,11 +31,76 @@
             {{ "checked" if custom_domain.catch_all else "" }}>
 
         <span class="custom-switch-indicator"></span>
+        <spam class="ml-2">
+          Catch All
+        </spam>
+
       </label>
     </form>
+    <div class="">
+      Simply use <b>anything@{{ custom_domain.domain }}</b>
+      next time you need an alias: it'll be <b>automatically</b>
+      created the first time it receives an email.
+      To have more fine-grained control, you can also use the
+      <a data-toggle="collapse" href="#regex-section">regular expression <i class="fe fe-chevrons-down"></i></a>.
+    </div>
+
+    <div class="{% if custom_domain.auto_create_regex is none %} collapse {% endif %}
+              {% if custom_domain.catch_all %} disabled-content {% endif %}
+            border border-info p-2"
+         id="regex-section">
+      <span class="badge badge-info">Advanced</span> <br>
+      You can also set a regular expression (regex): if an alias matches the expression, it'll be automatically created.
+      <br>
+      Please note that only the local part of the alias (i.e. <b>@{{ custom_domain.domain }}</b> is ignored) during the
+      regex
+      test.
+
+      <form method="post" class="form-inline" data-parsley-validate>
+        <input type="hidden" name="form-name" value="set-auto_create_regex">
+        <div class="form-group">
+          <input class="form-control mr-2"
+                 value="{{ custom_domain.auto_create_regex or "" }}"
+                 name="auto_create_regex"
+                 required
+                 data-parsley-pattern="[0-9a-z-_.(\\)(\*)]{1,}"
+                 data-parsley-trigger="change"
+                 data-parsley-error-message="Only lowercase letter, number, dot (.), dash (-), underscore (_), backslash (\), star (*)  are currently supported."
+                 placeholder="prefix\..*">
+        </div>
+        <button class="btn btn-outline-primary" name="action" value="save">Save</button>
+        {% if custom_domain.auto_create_regex %}
+          <button class="btn btn-outline-danger float-right ml-2" name="action" value="remove">Remove</button>
+        {% endif %}
+      </form>
+
+      For example, if you want aliases that starts with <b>prefix.</b> to be automatically created, you can set the
+      regex to <em data-toggle="tooltip"
+                   title="Click to copy"
+                   class="clipboard"
+                   data-clipboard-text="prefix\..*">prefix\..*</em>
+      <br>
+
+      If you want aliases that ends with <b>.suffix</b> to be automatically created, you can use the regex
+      <em data-toggle="tooltip"
+          title="Click to copy"
+          class="clipboard"
+          data-clipboard-text=".*\.suffix">.*\.suffix</em>
+      <br>
+
+      To test out regex, we recommend using regex tester tool like <a href="https://regex101.com" target="_blank">https://regex101.com↗</a>
+    </div>
+
+    <div>
+      The new alias will belong to
+      {% for mailbox in custom_domain.mailboxes %}
+        <b>{{ mailbox.email }}</b>
+        {% if not loop.last %},{% endif %}
+      {% endfor %}
+    </div>
   </div>
 
-  <div class="{% if not custom_domain.catch_all %} disabled-content {% endif %}">
+  <div class="{% if not custom_domain.auto_create_alias_enabled %} disabled-content {% endif %}">
     <div>Auto-created aliases are automatically owned by these mailboxes</div>
     {% set domain_mailboxes=custom_domain.mailboxes %}
     <form method="post" class="mt-2">

app/dashboard/views/domain_detail.py

@@ -158,7 +158,7 @@ def domain_detail_dns(custom_domain_id):
 @dashboard_bp.route("/domains/<int:custom_domain_id>/info", methods=["GET", "POST"])
 @login_required
 def domain_detail(custom_domain_id):
-    custom_domain = CustomDomain.get(custom_domain_id)
+    custom_domain: CustomDomain = CustomDomain.get(custom_domain_id)
     mailboxes = current_user.mailboxes()
 
     if not custom_domain or custom_domain.user_id != current_user.id:
@@ -261,6 +261,26 @@ def domain_detail(custom_domain_id):
             return redirect(
                 url_for("dashboard.domain_detail", custom_domain_id=custom_domain.id)
             )
+        elif request.form.get("form-name") == "set-auto_create_regex":
+            if request.form.get("action") == "save":
+                auto_create_regex = request.form.get("auto_create_regex")
+                if auto_create_regex:
+                    custom_domain.auto_create_regex = auto_create_regex
+                    db.session.commit()
+                    flash("The auto create regex has been updated", "success")
+                else:
+                    flash("The auto create regex cannot be empty", "error")
+            else:
+                custom_domain.auto_create_regex = None
+                db.session.commit()
+                flash(
+                    f"The auto create regex has been has been removed",
+                    "info",
+                )
+            return redirect(
+                url_for("dashboard.domain_detail", custom_domain_id=custom_domain.id)
+            )
+
         elif request.form.get("form-name") == "delete":
             name = custom_domain.domain
             LOG.d("Schedule deleting %s", custom_domain)

app/models.py

@@ -1848,6 +1848,10 @@ class CustomDomain(db.Model, ModelMixin):
 
     user = db.relationship(User, foreign_keys=[user_id])
 
+    @property
+    def auto_create_alias_enabled(self) -> bool:
+        return self.catch_all or self.auto_create_regex is not None
+
     @property
     def mailboxes(self):
         if self._mailboxes:

static/style.css

@@ -169,4 +169,8 @@ textarea.parsley-error {
 
 .domain_detail_content {
     font-size: 15px;
+}
+
+.domain_detail_content .parsley-errors-list {
+    max-width: 20em;
 }