I had to replace the default underscore teplating delimiters/Interpolate regex for compatibility with asp.net webforms.From the website i opted for mustache like syntax
As far as you hasn't overwritten the Underscore.templateSettings.evaluate setting you should still can use: <% ... %> for evaluate code.
I have seen attempts to also define the Underscore.templateSettings.evaluate setting to match with Mustache syntax but Mustache fights very strong against template evaluations so is gonna be difficult.