Error
Error Code:
3686
MySQL Error 3686: Regex Index Out of Bounds
Description
MySQL Error 3686, 'Index out of bounds in regular expression search,' indicates that a regular expression function tried to access a capture group or match position that does not exist. This typically occurs when the specified index for a substring, occurrence, or group is invalid or beyond the range produced by the regex pattern. It signifies an attempt to reference a non-existent part of the regex match result.
Error Message
Index out of bounds in regular expression search.
Known Causes
3 known causesNon-existent Capture Group
Attempting to retrieve a capture group index (e.g., in REGEXP_SUBSTR or REGEXP_REPLACE) that was not defined or matched by the regular expression pattern.
Invalid Match Position or Occurrence
Specifying a start position (pos) or occurrence number (occurrence) parameter that is zero, negative, or exceeds the valid range for the target string or number of matches.
Mismatched Pattern and Index Expectation
The regular expression pattern itself might not create the expected number of capture groups, leading to an out-of-bounds error when a higher index is requested.
Solutions
4 solutions available1. Simplify the Regular Expression easy
The most common cause is an overly complex or malformed regular expression that exceeds internal limits.
1
Review the regular expression being used in your query.
2
Break down complex regex into simpler parts or use alternative string matching functions if possible. For example, instead of a complex regex for 'starts with X and ends with Y', consider `LIKE 'X%' AND LIKE '%Y'` or a combination of `SUBSTRING` and `RIGHT`.
SELECT * FROM your_table WHERE your_column REGEXP 'your_complex_regex';
-- Consider simplifying to:
SELECT * FROM your_table WHERE your_column LIKE 'your_start_pattern%' AND your_column LIKE '%your_end_pattern%';
-- Or if applicable:
SELECT * FROM your_table WHERE SUBSTRING(your_column, 1, LENGTH('your_start_pattern')) = 'your_start_pattern' AND RIGHT(your_column, LENGTH('your_end_pattern')) = 'your_end_pattern';3
Test the simplified regex to ensure it still achieves the desired filtering.
2. Increase Regex Limits (Advanced) advanced
In rare cases, the default limits for regex processing might be too low for legitimate, albeit complex, patterns.
1
Identify the relevant MySQL system variables. These are typically `regex_stack_limit` and `regex_time_limit`. The `regex_stack_limit` controls the maximum depth of the recursion stack used by the regex engine, and `regex_time_limit` sets a timeout for regex operations.
2
Temporarily increase these limits for the current session to see if it resolves the issue. This is a good diagnostic step.
SET SESSION regex_stack_limit = 1000000; -- Example value, adjust as needed
SET SESSION regex_time_limit = 1000; -- Example value in milliseconds, adjust as needed3
If the session-level change works, consider making the change permanent by modifying your `my.cnf` or `my.ini` configuration file. Restart the MySQL server for permanent changes to take effect.
# Example entry in my.cnf/my.ini
[mysqld]
regex_stack_limit = 1000000
regex_time_limit = 10004
Be cautious when increasing these limits, as excessively high values can impact performance and stability.
3. Update MySQL Version medium
Older MySQL versions may have bugs or less robust regex implementations that have been fixed in newer releases.
1
Check your current MySQL server version.
SELECT VERSION();2
Consult the MySQL release notes for your current and potential target versions to see if there are any known fixes related to regex processing or error 3686.
3
Plan and execute an upgrade to a more recent, stable version of MySQL. Always perform backups before any major version upgrade.
4. Review Data for Unusual Characters or Patterns easy
Sometimes, the data itself can cause unexpected behavior in regex processing, especially if it contains control characters or extremely long sequences that interact poorly with the regex engine.
1
Examine a sample of the data in the column being searched.
SELECT your_column FROM your_table LIMIT 100;2
Look for any unusual characters, very long strings, or patterns that might be indirectly related to the regex but are not explicitly part of it.
3
If problematic data is found, consider cleaning or sanitizing it. This might involve using `REPLACE` or `TRIM` functions, or a more complex data cleansing process.
UPDATE your_table SET your_column = REPLACE(your_column, '\n', ' '); -- Example: replace newline characters4
If the issue is with very long strings, consider if those strings are truly necessary for regex matching or if they can be truncated or processed differently.