Cache compiled Regex instances in Partition and RegionEndpoint for en…

[muhammad-othman]

Description

• Changed _partitionsByRegex dictionary key from string to Regex. The compiled Regex instances are now the keys themselves — no separate cache needed. This is thread-safe because the dictionary is already guarded by ReaderWriterLockSlim.

• Updated PartitionsTemplate.tt to generate new Regex(@"pattern", RegexOptions.Compiled) keys so future regeneration produces the correct code.

• Merged _allPartitionRegionRegex (HashSet) and added a compiled Regex cache into a single _partitionRegionRegexMap (Dictionary<string, Regex>). Also changed from .Match().Success to .IsMatch() to avoid unnecessary Match object allocation.

Motivation and Context

DOTNET-8584

Testing

Created a BenchmarkDotNet tests and got these results for Fn.Partition()

Method	Before	After
Known region (us-east-1)	241 ns	208 ns
Regex match (us-west-99)	308 ns	297 ns
Regex match (cn-south-1)	537 ns	260 ns
Regex match (us-gov-east-99)	1,282 ns	534 ns
No match (unknown-1)	759 ns	412 ns

RegionEndpoint.GetBySystemName: Shows no benchmark difference because it self-caches resolved regions in _hashBySystemName — subsequent calls hit the dictionary directly. The regex caching improvement applies only to the first resolution of an unknown region.

Dry-runs

• DotNet Dry-run ID: 88936fb2-0de8-4dac-baf1-9d358809e919
  • Pending
  • Completed successfully
  • Failed
• PowerShell Dry-run ID: d9576ae0-8a36-40cb-ad18-90074dfdef94
  • Pending
  • Completed successfully
  • Failed

Breaking Changes Assessment

1. Identify all breaking changes including the following details:
   • What functionality was changed?
   • How will this impact customers?
   • Why does this need to be a breaking change and what are the most notable non-breaking alternatives?
   • Are best practices being followed?
   • How have you tested this breaking change?
2. Has a senior/+ engineer been assigned to review this PR?

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license

[Copilot]

Pull request overview

Improves endpoint/partition resolution performance in the AWS .NET SDK Core by caching compiled Regex instances used to map regions to partitions and to match partition region patterns.

Changes:

• Cache compiled partition-region regexes in RegionEndpoint and use IsMatch() to avoid Match allocations.
• Change Partition’s _partitionsByRegex to key on compiled Regex instances (and update generated partition initialization accordingly).
• Update the partitions T4 template so regenerated code continues to emit compiled Regex usage; add a dev config entry for a Core patch release.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sdk/src/Core/RegionEndpoint/RegionEndpoint.cs	Replaces pattern set with a pattern→compiled-regex map and uses cached Regex.IsMatch() during unknown-region partition inference.
sdk/src/Core/Amazon.Runtime/Internal/Endpoints/StandardLibrary/Partition.cs	Stores compiled Regex instances for partition matching and iterates compiled regexes during partition lookup.
sdk/src/Core/Amazon.Runtime/Internal/Endpoints/StandardLibrary/Partition.generated.cs	Updates generated partition initialization to add compiled Regex objects as keys.
generator/ServiceClientGeneratorLib/Generators/Endpoints/PartitionsTemplate.tt	Adjusts template to generate compiled Regex keys going forward.
generator/ServiceClientGeneratorLib/Generators/Endpoints/PartitionsTemplate.cs	Regenerated T4 output reflecting the template change.
generator/.DevConfigs/1b22ae51-b2c9-40ca-bb2b-d16a341993fe.json	Adds Core dev config entry marking the change as a patch.