private static String stripBraces(String expression) {
    if (expression == null) {
      return null;
    }
    if (expression.startsWith("{") && expression.endsWith("}")) {
      return expression.substring(1, expression.length() - 1);
    }
    return expression;
  }
A pattern for matching possible variable names.

<p>This pattern accepts characters allowed in RFC 6570 Section 2.3 It also allows the
characters feign has allowed in the past "[]-$"

<p>The RFC specifies that a variable name followed by a ':' should be a max-length
specification. Feign deviates from the rfc in that the ':' value modifier is used to mark a
regular expression.
core/src/main/java/feign/template/Expressions.java