How to fix Error 573 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 573 – Regular Expression Too Complex

Why This Error Happens

Error 573 occurs when SQL Server’s built-in regular expression engine (used internally by functions like LIKE with complex wildcard patterns, or T-SQL regex-based operations) hits an internal complexity threshold while parsing a pattern string. This typically happens with patterns containing excessive nested wildcards, overly long strings, or a high number of alternations that cause the regex evaluator’s internal state machine to exceed its processing limits. It is not a corruption issue but rather a query-design problem where the pattern itself is too complex for the engine to evaluate efficiently within its safety bounds.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Query and Pattern

  • Check the SQL Server Error Log (C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG) for the full text of the failing statement.
  • Use SQL Server Profiler or Extended Events to capture the exact query executing at the time of the error.
  • Look specifically for LIKE, PATINDEX, or sys.dm_* calls using long wildcard strings (e.g., %value1%value2%value3%...).

Step 2: Simplify the Regular Expression Pattern

  • Break a single complex pattern into multiple simpler LIKE conditions joined by AND/OR instead of one long chained wildcard string.
  • Replace nested %...%...% chains with CHARINDEX() or PATINDEX() calls for more predictable performance.
  • Example rewrite:
  -- Instead of:
  WHERE column LIKE '%a%b%c%d%e%f%'
  -- Use:
  WHERE column LIKE '%a%' AND column LIKE '%b%' AND column LIKE '%c%'

Step 3: Increase Query Complexity Tolerance (If Applicable)

  • Review and adjust the MAXRECURSION and query timeout settings if the pattern is part of a recursive CTE feeding into the LIKE clause.
  • If using SQL Server 2025’s enhanced regex functions (REGEXP_LIKE, REGEXP_REPLACE), reduce backtracking-heavy constructs such as nested quantifiers (a+)+.
  • Apply OPTION (MAXDOP 1) temporarily to rule out parallel plan interference with pattern evaluation.

Step 4: Update SQL Server Cumulative Updates

  • Navigate to Control Panel > Programs > Programs and Features on Windows Server 2025 / Windows 11 to confirm the current CU build.
  • Download and apply the latest Cumulative Update for SQL Server 2025 from the Microsoft Update Catalog, as regex engine limits are periodically tuned in servicing releases.
  • Restart the SQL Server service via services.msc after installation.

Alternative or Verification Steps

  • Re-run the original query after simplification and confirm Error 573 no longer appears in the ERRORLOG or client output.
  • As an alternative, offload complex pattern matching to a CLR function or external application layer (e.g., a .NET regex library) instead of relying on native T-SQL wildcard evaluation for very long or deeply nested patterns.