@Override
  public String getValue() {
    if (this.pattern != null) {
      return "{" + this.name + ":" + this.pattern + "}";
    }
    return "{" + this.name + "}";
  }
Checks if the provided value matches the variable pattern, if one is defined. Always true if no
pattern is defined.

@param value to check.
@return true if it matches.
core/src/main/java/feign/template/Expression.java