Back in March 2025 when Microsoft first announced that REGEX support was coming to SQL Server 2025 and Azure SQL DB, I gave it a quick test, and the performance was horrific. It was bad in 3 different ways:
The CPU usage was terrible, burning 60 seconds of CPU time to check a few million rows It refused to use an index The cardinality estimation was terrible, hard-coded to 30% of the table
Prompted by a comment from Erland Sommarskog this month, I circled back and ran the tests again with the release version of SQL Server 2025. Great news!