Allow underscores in SMS senders

app/main/forms.py

@@ -46,7 +46,7 @@ from app.main.validators import (
     Blacklist,
     CsvFileValidator,
     DoesNotStartWithDoubleZero,
-    LettersNumbersAndFullStopsOnly,
+    LettersNumbersFullStopsAndUnderscoresOnly,
     MustContainAlphanumericCharacters,
     NoCommasInPlaceHolders,
     NoEmbeddedImagesInSVG,
@@ -1086,7 +1086,7 @@ class ServiceSmsSenderForm(StripWhitespaceForm):
             DataRequired(message="Cannot be empty"),
             Length(max=11, message="Enter 11 characters or fewer"),
             Length(min=4, message="Enter 4 characters or more"),
-            LettersNumbersAndFullStopsOnly(),
+            LettersNumbersFullStopsAndUnderscoresOnly(),
             DoesNotStartWithDoubleZero(),
         ]
     )

app/main/validators.py

@@ -103,9 +103,9 @@ class OnlySMSCharacters:
             )
 
 
-class LettersNumbersAndFullStopsOnly:
+class LettersNumbersFullStopsAndUnderscoresOnly:
 
-    regex = re.compile(r'^[a-zA-Z0-9\s\.]+$')
+    regex = re.compile(r'^[a-zA-Z0-9\s\._]+$')
 
     def __init__(self, message='Use letters and numbers only'):
         self.message = message

tests/app/main/test_validators.py

@@ -215,6 +215,16 @@ def test_sms_sender_form_validation(
     form.validate()
     assert 'Use letters and numbers only' == form.errors['sms_sender'][0]
 
+    # Underscores are allowed
+    form.sms_sender.data = 'UK_GOV'
+    form.validate()
+    assert not form.errors
+
+    # Full stops are allowed
+    form.sms_sender.data = 'UK.GOV'
+    form.validate()
+    assert not form.errors
+
     form.sms_sender.data = '333'
     form.validate()
     assert 'Enter 4 characters or more' == form.errors['sms_sender'][0]