Condition if "contains"

<% if (context.profile.firstName == "Charly" || context.profile.firstName == "CHARLES") { %>You are my favorite Charles in the world!<% } else { %><%= context.profile.firstName %>, you are my favourite boy.<%} %>

I have the condition above for a subject in Adobe Campaign Standard, and I would need to replace the two conditions with one that said; x contains “charl” (not case sensitive). String.includes and .includes() don't work. Any ideas?