The PathMatcher DSL

The PathMatcher mini-DSL is used to match incoming URL’s and extract values from them. It is used in the path directive.

Some simple examples of the DSL in action can be found below.

Overview

When a request (or rather the respective RequestContextRequestContext instance) enters the route structure it has an “unmatched path” that is identical to the request.uri.path. As it descends the routing tree and passes through one or more pathPrefix or path directives the “unmatched path” progressively gets “eaten into” from the left until, in most cases, it eventually has been consumed completely.

What exactly gets matched and consumed as well as extracted from the unmatched path in each directive is defined with the path matching DSL, which is built around these types:

Scala
trait PathMatcher[L: Tuple]
type PathMatcher0 = PathMatcher[Unit]
type PathMatcher1[T] = PathMatcher[Tuple1[T]]
type PathMatcher2[T,U] = PathMatcher[Tuple2[T,U]]
// .. etc
Java
package akka.http.javadsl.server;
class PathMatcher0
class PathMatcher1<T1>
class PathMatcher2<T1, T2>
// .. etc

The number and types of the values extracted by a PathMatcher instance is represented by the L type parameter which needs to be one of Scala’s TupleN types or Unit (which is designated by the Tuple context bound). is determined by the class and its type parameters. The convenience alias PathMatcher0 can be used for all matchers which don’t extract anything while PathMatcher1[T] defines a matcher which only extracts a single value of type T.

Here is an example of a more complex PathMatcher expression:

Scala
val matcher: PathMatcher1[Option[Int]] =
  "foo" / "bar" / "X" ~ IntNumber.? / ("edit" | "create")

val route: Route =
  path(matcher) { i: Option[Int] =>
    complete(s"Matched X${i.getOrElse("")}")
  }
Java
import akka.http.javadsl.server.PathMatcher1;
import static akka.http.javadsl.server.PathMatchers.segment;
import static akka.http.javadsl.server.PathMatchers.integerSegment;

PathMatcher1<Integer> matcher =
  PathMatchers
    .segment("foo")
    .slash("bar")
    .slash(
      segment("X").concat(integerSegment())
    )
    .slash(
      segment("edit").orElse(segment("create"))
    );

Route route = path(matcher, i ->
  complete("Matched X" + i)
);

This will match paths like foo/bar/X42/edit or foo/bar/X/createfoo/bar/X37/create.

Note

The path matching DSL describes what paths to accept after URL decoding. This is why the path-separating slashes have special status and cannot simply be specified as part of a string! The string “foo/bar” would match the raw URI path “foo%2Fbar”, which is most likely not what you want!

Basic PathMatchers

A complex PathMatcher can be constructed by combining or modifying more basic ones. Here are the basic matchers that Akka HTTP already provides for you:

String
You can use a String instance as a PathMatcher0. Strings simply match themselves and extract no value. Note that strings are interpreted as the decoded representation of the path, so if they include a ‘/’ character this character will match “%2F” in the encoded raw URI!
Regex
You can use a Regex instance as a PathMatcher1[String], which matches whatever the regex matches and extracts one String value. A PathMatcher created from a regular expression extracts either the complete match (if the regex doesn’t contain a capture group) or the capture group (if the regex contains exactly one capture group). If the regex contains more than one capture group an IllegalArgumentException will be thrown.
Map[String, T]
You can use a Map[String, T] instance as a PathMatcher1[T], which matches any of the keys and extracts the respective map value for it.
Slash: PathMatcher0
Matches exactly one path-separating slash (/) character and extracts nothing.
Segment: PathMatcher1[String]
Matches if the unmatched path starts with a path segment (i.e. not a slash). If so the path segment is extracted as a String instance.
PathEnd: PathMatcher0
Matches the very end of the path, similar to $ in regular expressions and extracts nothing.
Remaining: PathMatcher1[String]
Matches and extracts the complete remaining unmatched part of the request’s URI path as an (encoded!) String. If you need access to the remaining decoded elements of the path use RemainingPath instead.
RemainingPath: PathMatcher1[Path]
Matches and extracts the complete remaining, unmatched part of the request’s URI path.
IntNumber: PathMatcher1[Int]
Efficiently matches a number of decimal digits (unsigned) and extracts their (non-negative) Int value. The matcher will not match zero digits or a sequence of digits that would represent an Int value larger than Int.MaxValue.
LongNumber: PathMatcher1[Long]
Efficiently matches a number of decimal digits (unsigned) and extracts their (non-negative) Long value. The matcher will not match zero digits or a sequence of digits that would represent an Long value larger than Long.MaxValue.
HexIntNumber: PathMatcher1[Int]
Efficiently matches a number of hex digits and extracts their (non-negative) Int value. The matcher will not match zero digits or a sequence of digits that would represent an Int value larger than Int.MaxValue.
HexLongNumber: PathMatcher1[Long]
Efficiently matches a number of hex digits and extracts their (non-negative) Long value. The matcher will not match zero digits or a sequence of digits that would represent an Long value larger than Long.MaxValue.
DoubleNumber: PathMatcher1[Double]
Matches and extracts a Double value. The matched string representation is the pure decimal, optionally signed form of a double value, i.e. without exponent.
JavaUUID: PathMatcher1[UUID]
Matches and extracts a java.util.UUID instance.
Neutral: PathMatcher0
A matcher that always matches, doesn’t consume anything and extracts nothing. Serves mainly as a neutral element in PathMatcher composition.
Segments: PathMatcher1[List[String]]
Matches all remaining segments as a list of strings. Note that this can also be “no segments” resulting in the empty list. If the path has a trailing slash this slash will not be matched, i.e. remain unmatched and to be consumed by potentially nested directives.
separateOnSlashes(string: String): PathMatcher0
Converts a path string containing slashes into a PathMatcher0 that interprets slashes as path segment separators. This means that a matcher matching “%2F” cannot be constructed with this helper.
provide[L: Tuple](extractions: L): PathMatcher[L]
Always matches, consumes nothing and extracts the given TupleX of values.
PathMatcher[L: Tuple](prefix: Path, extractions: L): PathMatcher[L]
Matches and consumes the given path prefix and extracts the given list of extractions. If the given prefix is empty the returned matcher matches always and consumes nothing.

A path matcher is a description of a part of a path to match. The simplest path matcher is PathMatcher.segment which matches exactly one path segment against the supplied constant string.

Other path matchers defined in PathMatchersPathMatchers match the end of the path (PathMatchers.END), a single slash (PathMatchers.SLASH), or nothing at all (PathMatchers.NEUTRAL).

Many path matchers are hybrids that can both match (by using them with one of the PathDirectives) and extract values, Extracting a path matcher value (i.e. using it with handleWithX) is only allowed if it nested inside a path directive that uses that path matcher and so specifies at which position the value should be extracted from the path.

Predefined path matchers allow extraction of various types of values:

PathMatchers.segment(String)
Strings simply match themselves and extract no value. Note that strings are interpreted as the decoded representation of the path, so if they include a ‘/’ character this character will match “%2F” in the encoded raw URI!
PathMatchers.segment(java.util.regex.Pattern)
You can use a regular expression instance as a path matcher, which matches whatever the regex matches and extracts one String value. A PathMatcher created from a regular expression extracts either the complete match (if the regex doesn’t contain a capture group) or the capture group (if the regex contains exactly one capture group). If the regex contains more than one capture group an IllegalArgumentException will be thrown.
PathMatchers.SLASH
Matches exactly one path-separating slash (/) character.
PathMatchers.END
Matches the very end of the path, similar to $ in regular expressions.
PathMatchers.Segment
Matches if the unmatched path starts with a path segment (i.e. not a slash). If so the path segment is extracted as a String instance.
PathMatchers.Remaining
Matches and extracts the complete remaining unmatched part of the request’s URI path as an (encoded!) String. If you need access to the remaining decoded elements of the path use RemainingPath instead.
PathMatchers.intValue
Efficiently matches a number of decimal digits (unsigned) and extracts their (non-negative) Int value. The matcher will not match zero digits or a sequence of digits that would represent an Int value larger than Integer.MAX_VALUE.
PathMatchers.longValue
Efficiently matches a number of decimal digits (unsigned) and extracts their (non-negative) Long value. The matcher will not match zero digits or a sequence of digits that would represent an Long value larger than Long.MAX_VALUE.
PathMatchers.hexIntValue
Efficiently matches a number of hex digits and extracts their (non-negative) Int value. The matcher will not match zero digits or a sequence of digits that would represent an Int value larger than Integer.MAX_VALUE.
PathMatchers.hexLongValue
Efficiently matches a number of hex digits and extracts their (non-negative) Long value. The matcher will not match zero digits or a sequence of digits that would represent an Long value larger than Long.MAX_VALUE.
PathMatchers.uuid
Matches and extracts a java.util.UUID instance.
PathMatchers.NEUTRAL
A matcher that always matches, doesn’t consume anything and extracts nothing. Serves mainly as a neutral element in PathMatcher composition.
PathMatchers.segments
Matches all remaining segments as a list of strings. Note that this can also be “no segments” resulting in the empty list. If the path has a trailing slash this slash will not be matched, i.e. remain unmatched and to be consumed by potentially nested directives.

Combinators

Path matchers can be combined with these combinators to form higher-level constructs:

Tilde Operator (~)
The tilde is the most basic combinator. It simply concatenates two matchers into one, i.e if the first one matched (and consumed) the second one is tried. The extractions of both matchers are combined type-safely. For example: "foo" ~ "bar" yields a matcher that is identical to "foobar".
Slash Operator (/)
This operator concatenates two matchers and inserts a Slash matcher in between them. For example: "foo" / "bar" is identical to "foo" ~ Slash ~ "bar".
Pipe Operator (|)
This operator combines two matcher alternatives in that the second one is only tried if the first one did not match. The two sub-matchers must have compatible types. For example: "foo" | "bar" will match either “foo” or “bar”. When combining an alternative expressed using this operator with an / operator, make sure to surround the alternative with parentheses, like so: ("foo" | "bar") / "bom". Otherwise, the / operator takes precedence and would only apply to the right-hand side of the alternative.

Modifiers

Path matcher instances can be transformed with these modifier methods:

/
The slash operator cannot only be used as combinator for combining two matcher instances, it can also be used as a postfix call. matcher / is identical to matcher ~ Slash but shorter and easier to read.

? : By postfixing a matcher with ? you can turn any PathMatcher into one that always matches, optionally consumes and potentially extracts an Option of the underlying matchers extraction. The result type depends on the type of the underlying matcher:

If a matcher is of type then matcher.? is of type
PathMatcher0 PathMatcher0
PathMatcher1[T] PathMatcher1[Option[T]]
PathMatcher[L: Tuple] PathMatcher[Option[L]]

repeat(separator: PathMatcher0 = PathMatchers.Neutral) : By postfixing a matcher with repeat(separator) you can turn any PathMatcher into one that always matches, consumes zero or more times (with the given separator) and potentially extracts a List of the underlying matcher’s extractions. The result type depends on the type of the underlying matcher:

If a matcher is of type then matcher.repeat(...) is of type
PathMatcher0 PathMatcher0
PathMatcher1[T] PathMatcher1[List[T]]
PathMatcher[L: Tuple] PathMatcher[List[L]]
unary_!
By prefixing a matcher with ! it can be turned into a PathMatcher0 that only matches if the underlying matcher does not match and vice versa.
transform / (h)flatMap / (h)map
These modifiers allow you to append your own “post-application” logic to another matcher in order to form a custom one. You can map over the extraction(s), turn mismatches into matches or vice-versa or do anything else with the results of the underlying matcher. Take a look at the method signatures and implementations for more guidance as to how to use them.

Examples

Here’s a collection of path matching examples:

Scala
// matches /foo/
path("foo"./)

// matches /foo/bar
path("foo" / "bar")

// NOTE: matches /foo%2Fbar and doesn't match /foo/bar
path("foo/bar")

// matches /foo/bar
path(separateOnSlashes("foo/bar"))

// matches e.g. /foo/123 and extracts "123" as a String
path("foo" / """\d+""".r)

// matches e.g. /foo/bar123 and extracts "123" as a String
path("foo" / """bar(\d+)""".r)

// similar to `path(Segments)`
path(Segment.repeat(10, separator = Slash))

// matches e.g. /i42 or /hCAFE and extracts an Int
path("i" ~ IntNumber | "h" ~ HexIntNumber)

// identical to path("foo" ~ (PathEnd | Slash))
path("foo" ~ Slash.?)

// matches /red or /green or /blue and extracts 1, 2 or 3 respectively
path(Map("red" -> 1, "green" -> 2, "blue" -> 3))

// matches anything starting with "/foo" except for /foobar
pathPrefix("foo" ~ !"bar")
Java
import static akka.http.javadsl.server.Directives.complete;
import static akka.http.javadsl.server.Directives.path;
import static akka.http.javadsl.server.Directives.pathEnd;
import static akka.http.javadsl.server.Directives.pathPrefix;
import static akka.http.javadsl.server.Directives.pathSingleSlash;
import static akka.http.javadsl.server.Directives.route;

// matches "/test"
path("test", () ->
  complete(StatusCodes.OK)
);

// matches "/test", as well
path(PathMatchers.segment("test"), () ->
  complete(StatusCodes.OK)
);

// matches "/admin/user"
path(PathMatchers.segment("admin")
  .slash("user"), () ->
  complete(StatusCodes.OK)
);

// matches "/admin/user", as well
pathPrefix("admin", () ->
  path("user", () ->
    complete(StatusCodes.OK)
  )
);

// matches "/admin/user/<user-id>"
path(PathMatchers.segment("admin")
  .slash("user")
  .slash(PathMatchers.integerSegment()), userId -> {
    return complete("Hello user " + userId);
  }
);

// matches "/admin/user/<user-id>", as well
pathPrefix("admin", () ->
  path("user", () ->
    path(PathMatchers.integerSegment(), userId ->
      complete("Hello user " + userId)
    )
  )
);

// never matches
path("admin", () -> // oops this only matches "/admin", and no sub-paths
  path("user", () ->
    complete(StatusCodes.OK)
  )
);

// matches "/user/" with the first subroute, "/user" (without a trailing slash)
// with the second subroute, and "/user/<user-id>" with the last one.
pathPrefix("user", () -> concat(
  pathSingleSlash(() ->
    complete(StatusCodes.OK)
  ),
  pathEnd(() ->
    complete(StatusCodes.OK)
  ),
  path(PathMatchers.integerSegment(), userId ->
    complete("Hello user " + userId)
  )
));
Found an error in this documentation? The source code for this page can be found here. Please feel free to edit and contribute a pull request.