Declarative String Processing for Swift

An early experimental general-purpose pattern matching engine for Swift.

See Declarative String Processing Overview

Requirements

• Currently tested with Swift Trunk Development Snapshot DEVELOPMENT-SNAPSHOT-2025-09-14 or later.

Trying it out

To try out the functionality provided here, download the latest open source development toolchain. Import _StringProcessing in your source file to get access to the API and specify -Xfrontend -enable-experimental-string-processing to get access to the literals.

For example, in a Package.swift file's target declaration:

.target(
    name: "foo",
    dependencies: ["depA"],
    swiftSettings: [.unsafeFlags(["-Xfrontend", "-enable-experimental-string-processing"])]
 ),

Integration with Swift

_RegexParser and _StringProcessing are specially integrated modules that are built as part of apple/swift.

Specifically, _RegexParser contains the parser for regular expression literals and is built both as part of the compiler and as a core library. _CUnicode and _StringProcessing are built together as a core library named _StringProcessing.

Branching scheme

The main branch is the branch for day-to-day development. Generally, you should create PRs against this branch.

Branches whose name starts with swift/ are Swift integration branches similar to those in apple/llvm-project. For each branch, dropping the swift/ prefix is the corresponding branch in apple/swift.

This package's main branch automatically integrates with Swift's main branch.

A pair of corresponding branches are expected to build successfully together and pass all tests.

Integration workflow

To integrate the latest changes in apple/swift-experimental-string-processing to apple/swift, carefully follow the workflow:

• Run package CI
  • In the pull request, trigger package CI using

@swift-ci please test

• Run full Swift CI for any changes to public or SPI interfaces or the _RegexParser module.
  • If apple/swift needs to be modified to work with the latest main in apple/swift-experimental-string-processing, create a pull request in apple/swift. Note: Since CI in apple/swift-experimental-string-processing has not yet been set up to run full toolchain tests, you should create a PR in apple/swift regardless; if the integration does not require changing apple/swift, create a dummy PR in apple/swift by changing the README and close it in the end instead of merging.
  • In the apple/swift-experimental-string-processing pull request, trigger CI using the following command (replacing <PR NUMBER> with the apple/swift pull request number, if any):

    apple/swift#<PR NUMBER> # use this line only if there is an corresponding apple/swift PR
    @swift-ci please test
    

  • In the apple/swift pull request (if any), trigger CI using the following command (replacing <PR NUMBER> with the apple/swift-experimental-string-processing pull request number):

    apple/swift-experimental-string-processing#<PR NUMBER>
    @swift-ci please test
    

• Merge when approved.
  • Merge the PR in apple/swift-experimental-string-processing:
    • as a squash or rebase if against main (the development branch).
    • as a merge commit if it's a merge from main to swift/release/x.y.
  • Merge the pull request in apple/swift (if any).

Development notes

Compiler integration can be tricky. Use special caution when developing _RegexParser and _StringProcessing modules.

• Do not change the names of these modules without due approval from compiler and infrastructure teams.
• Do not modify the existing ABI (e.g. C API, serialization format) between the regular expression parser and the Swift compiler unless absolutely necessary.
• Always minimize the number of lockstep integrations, i.e. when apple/swift-experimental-string-processing and apple/swift have to change together. Whenever possible, introduce new API first, migrate Swift compiler onto it, and then deprecate old API. Use versioning if helpful.
• In _StringProcessing, do not write fully qualified references to symbols in _CUnicode, and always wrap import _CUnicode in a #if canImport(_CUnicode). This is because _CUnicode is built as part of _StringProcessing with CMake.