rocket::build()
    .attach(Template::custom(|engines| {
        engines.tera.register_filter("url", {
            use regex::{Captures, Regex};
            use rocket_dyn_templates::tera::{Error, Value};
            Box::new(
                move |v: &Value,
                        _: &std::collections::HashMap<String, Value>|
                        -> Result<Value, Error> {
                    match v.as_str() {
                        Some(s) => match Regex::new(r"[A-z]+:\/\/(?:[^\s]+|$)") {
                            Ok(r) => Ok(r
                                .replace_all(s, |c: &Captures| {
                                    format!("<a href=\"{}\">{}</a>", &c[0], &c[0])
                                })
                                .into()),
                            Err(e) => Err(Error::msg(e)),
                        },
                        None => Err(Error::msg("Expected a string")),
                    }
                },
            )
        })
    }))
    ...