Language Syntax for Text Manipulation in dmx binding Statements

I have a variable called display_mode which I use to contol the visibility of various windows in my app. So it can have values such as:

app_contacts
app_finances
login_signup
login_login

For certain part of my app, I want to access the text section of display_mode which is before the underscore, so to find “app” or “login”.

So I’m looking for syntax along the lines of:

display_mode.text_before('_')

What syntax do I use for that?