Regex pattern attribute causing error

I have a big html snippet generated by a tool ( http://mbylstra.github.io/html-to-elm/ ) and it took me a long time to discover what was crashing the elm-format.

Then I discovered that inside `Html.Attribute.pattern "[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,3}$"` the character \ inside the string was crashing and it was almost impossible to discover because the error was showing at the beginning of the view function.

SSCCE:
https://ellie-app.com/NHJHZk7d44a1

Start by reproducing the crash with the SSCCE linked in the issue, focusing on the Html.Attribute.pattern string and its escaped character. Trace the formatter entry point that handles this expression; done means elm-format processes the snippet without crashing and reports errors at the relevant location.

Written by the indexing model from the issue text.