Error
Error Code:
3688
MySQL Error 3688: Regular Expression Syntax Error
Description
This error indicates an issue with the syntax of a regular expression used within a MySQL query or statement. It occurs when the database system cannot parse the provided regex pattern due to incorrect formatting or unsupported characters. The error message typically pinpoints the line and character where the syntax violation was detected.
Error Message
Syntax error in regular expression on line %u, character %u.
Known Causes
4 known causesUnmatched Parentheses or Brackets
The regular expression contains an opening parenthesis or bracket without a corresponding closing one, or vice-versa, leading to an incomplete pattern.
Incorrect Escape Characters
Special characters within the regex are escaped incorrectly, or an unknown escape sequence is used that MySQL's regex engine does not recognize.
Unsupported Regex Syntax
The regular expression uses syntax or features that are not supported by MySQL's specific regular expression engine (e.g., POSIX or specific PCRE features).
Misplaced Quantifiers
Quantifiers like *, +, ?, or {} are applied to an element that cannot be quantified or are placed in an invalid position within the pattern.
Solutions
3 solutions available1. Correct Invalid Regular Expression Characters easy
Identify and fix syntax errors in the regular expression string.
1
The error message 'Syntax error in regular expression on line %u, character %u' points to the exact location of the problem within your regular expression string. Carefully examine the character at the specified line and character position.
Example: If the error points to character 5 on line 1, and your regex is `^abc.def$`, the error might be with the `.` if it's intended to be a literal dot.2
Common issues include unescaped special characters that are intended to be literal. For example, if you want to match a literal dot (`.`), you need to escape it with a backslash (`\.`). Similarly, for characters like `*`, `+`, `?`, `^`, `$`, `(`, `)`, `[`, `]`, `{`, `}`, `|`, and `\`.
Incorrect: `SELECT * FROM my_table WHERE my_column REGEXP 'file.txt';`
Correct: `SELECT * FROM my_table WHERE my_column REGEXP 'file\.txt';`3
Ensure that character classes `[]` are properly formed. For example, a hyphen `-` within a character class has a special meaning for ranges and should be escaped (`\-`) if intended as a literal hyphen, or placed at the beginning or end of the class.
Incorrect: `SELECT * FROM my_table WHERE my_column REGEXP '[a-z-0-9]';` (Potential issue with the hyphen)
Correct: `SELECT * FROM my_table WHERE my_column REGEXP '[a-z\-0-9]';` or `SELECT * FROM my_table WHERE my_column REGEXP '[-a-z0-9]';`2. Use MySQL's REGEXP_LIKE Function for Clarity easy
Replace the REGEXP operator with the more explicit REGEXP_LIKE function, which can sometimes offer better error reporting or handling.
1
The `REGEXP_LIKE()` function provides a more structured way to perform regular expression matching. While it uses the same underlying regex engine, its syntax might be clearer and less prone to the direct operator interpretation issues.
Replace:
`SELECT * FROM my_table WHERE my_column REGEXP 'pattern';`
With:
`SELECT * FROM my_table WHERE REGEXP_LIKE(my_column, 'pattern');`2
When using `REGEXP_LIKE()`, ensure that the pattern string itself is correctly escaped for standard regular expression syntax. The error 3688 can still occur if the pattern within `REGEXP_LIKE()` is invalid.
Example of escaping within REGEXP_LIKE:
`SELECT * FROM my_table WHERE REGEXP_LIKE(my_column, '^user[0-9]+\.log$');`3. Validate Regular Expression Against a Standard medium
Test your regex against a known-good regex validator to pinpoint syntax issues.
1
Copy the regular expression string that is causing the error from your MySQL query.
Example regex: `^([0-9]{1,3}\.){3}[0-9]{1,3}$`2
Use an online regular expression validator. Websites like regex101.com or regexr.com are excellent tools. Select 'MySQL' as the regex flavor.
Navigate to https://regex101.com/3
Paste your regex into the validator. It will immediately highlight any syntax errors and explain what they are, often providing suggestions for correction.
Paste the regex into the 'Regular Expression' input field on regex101.com.4
Once you've identified and corrected the syntax error using the validator, re-apply the corrected regex to your MySQL query.
Corrected regex: `^([0-9]{1,3}\.){3}[0-9]{1,3}$` (if the validator found an issue with the escaping of the dot)