diff --git a/src/gui_qt/fit/function_creation_dialog.py b/src/gui_qt/fit/function_creation_dialog.py
index 4c104bd..41d9363 100644
--- a/src/gui_qt/fit/function_creation_dialog.py
+++ b/src/gui_qt/fit/function_creation_dialog.py
@@ -14,7 +14,7 @@ from gui_qt.lib.namespace import QNamespaceWidget
 
 __all__ = ['QUserFitCreator']
 
-validator = QtGui.QRegExpValidator(QtCore.QRegExp('[A-Za-z]\S*'))
+validator = QtGui.QRegExpValidator(QtCore.QRegExp('[_A-Za-z][_A-Za-z0-9]*'))
 pattern = re.compile(r'def func\(.*\):', flags=re.MULTILINE)
 
 
@@ -145,6 +145,7 @@ class QUserFitCreator(QtWidgets.QDialog, Ui_Dialog):
         self.classCreated.emit()
         super().accept()
 
+
 class KwargsWidget(QtWidgets.QWidget):
     Changed = QtCore.pyqtSignal()