don't allow commas in placeholder fields

app/main/forms.py

@@ -50,6 +50,7 @@ from app.main.validators import (
     CommonlyUsedPassword,
     CsvFileValidator,
     DoesNotStartWithDoubleZero,
+    FieldCannotContainComma,
     LettersNumbersSingleQuotesFullStopsAndUnderscoresOnly,
     MustContainAlphanumericCharacters,
     NoCommasInPlaceHolders,
@@ -1650,7 +1651,11 @@ def get_placeholder_form_instance(
             )  # TODO: replace with us_mobile_number
     else:
         field = GovukTextInputField(
-            placeholder_name, validators=[DataRequired(message="Cannot be empty")]
+            placeholder_name,
+            validators=[
+                DataRequired(message="Cannot be empty"),
+                FieldCannotContainComma(),
+            ],
         )
 
     PlaceholderForm.placeholder_value = field

app/main/validators.py

@@ -161,6 +161,15 @@ class DoesNotStartWithDoubleZero:
             raise ValidationError(self.message)
 
 
+class FieldCannotContainComma:
+    def __init__(self, message="Cannot contain a comma"):
+        self.message = message
+
+    def __call__(self, form, field):
+        if field.data and "," in field.data:
+            raise ValidationError(self.message)
+
+
 class MustContainAlphanumericCharacters:
     regex = re.compile(r".*[a-zA-Z0-9].*[a-zA-Z0-9].*")