@Nullable
    private static <T, U> RoutingPredicate<T> parseSingleRoutingPredicate(
            String predicateExpr, Function<String, U> nameConverter,
            Function<U, Predicate<T>> containsPredicate,
            BiFunction<U, String, Predicate<T>> equalsPredicate) {
        if (predicateExpr.contains("|")) {
            // `|` should appear only in the or-form such as `foo || bar`.
            return null;
        }
        final RoutingPredicate<T> routingPredicate;
        final RoutingPredicate<T> containRoutingPredicate =
                parseContainRoutingPredicate(predicateExpr, nameConverter, containsPredicate);
        if (containRoutingPredicate != null) {
            routingPredicate = containRoutingPredicate;
        } else {
            routingPredicate = parseCompareRoutingPredicate(predicateExpr, nameConverter, equalsPredicate);
        }
        return routingPredicate;
    }
Patterns used to parse a given predicate. The predicate can be one of the following forms:
<ul>
    <li>{@code some-name=some-value} which means that the request must have a
    {@code some-name=some-value} header or parameter</li>
    <li>{@code some-name!=some-value} which means that the request must not have a
    {@code some-name=some-value} header or parameter</li>
    <li>{@code some-name} which means that the request must contain a {@code some-name} header or
    parameter</li>
    <li>{@code !some-name} which means that the request must not contain a {@code some-name} header or
    parameter</li>
    <li>{@code some-name=some-value || some-other-value} which means that the request must have
    a {@code some-name=some-value} or a {@code some-other-value} header or parameter</li>
</ul>
core/src/main/java/com/linecorp/armeria/server/RoutingPredicate.java