📋

Description

Error 3691 indicates a syntax issue within a regular expression used in a MySQL query. It occurs when a `REGEXP` or `RLIKE` pattern contains an unbalanced number of opening and closing parentheses, making the expression syntactically invalid and preventing query execution.

💬

Error Message

Mismatched parenthesis in regular expression.

🔍

Known Causes

4 known causes

⚠️

Unclosed Opening Parenthesis

A regular expression contains an opening parenthesis `(` that does not have a corresponding closing parenthesis `)`. This often happens with grouping or non-capturing groups.

⚠️

Unmatched Closing Parenthesis

A closing parenthesis `)` exists in the regular expression without a preceding, unclosed opening parenthesis. This can occur from typos or incorrect pattern construction.

⚠️

Incorrect Parenthesis Escaping

Parentheses intended to be literal characters (`(` or `)`) were not properly escaped, causing the regex engine to interpret them as structural elements instead.

⚠️

Complex Pattern Imbalance

In highly complex or nested regular expressions, parentheses used for grouping, alternation, or character classes become imbalanced due to oversight or copy-paste errors.

🛠️

Solutions

3 solutions available

1. Correct Unbalanced Parentheses in Regex easy

Identify and fix unmatched parentheses within your regular expression patterns.

1

Carefully review the regular expression string used in your MySQL query (e.g., in `REGEXP`, `RLIKE`, `NOT REGEXP`, `NOT RLIKE` clauses, or functions like `REGEXP_REPLACE`, `REGEXP_SUBSTR`).

SELECT * FROM your_table WHERE your_column REGEXP 'your_regex_with_unbalanced_parentheses';

2

Look for pairs of opening `(` and closing `)` parentheses. Ensure every opening parenthesis has a corresponding closing parenthesis, and vice-versa. Pay close attention to nested parentheses.

Example of an invalid regex: `^([a-z]+)abc)$` (missing closing parenthesis for the outer group).
Example of a valid regex: `^([a-z]+)abc$` or `^((a|b)c)$`.

3

Correct the regex by adding or removing parentheses to balance them. If you intended to match a literal parenthesis, escape it with a backslash `\`.

Corrected regex for the invalid example above: `^([a-z]+)abc\)$` (if you want to match a literal closing parenthesis) or `^([a-z]+)abc$` (if the closing parenthesis was unintentional).

4

Re-run your MySQL query with the corrected regular expression.

SELECT * FROM your_table WHERE your_column REGEXP 'your_corrected_regex';

2. Escape Special Characters in Regex easy

Escape literal parentheses that are not intended as regex grouping or capturing operators.

1

If your regular expression needs to match literal parenthesis characters `(` or `)`, you must escape them with a backslash `\`.

Consider a regex that needs to match the string `(abc)`.
Incorrect regex: `(abc)` - This will be interpreted as a capturing group.
Correct regex: `\(abc\)`

2

Apply this escaping to all literal parentheses within your regex string.

SELECT * FROM your_table WHERE your_column REGEXP '\(some_pattern\)';

3

Test your query after escaping the special characters.

SELECT * FROM your_table WHERE your_column REGEXP '\\(literal_parenthesis\\)';

3. Simplify or Re-evaluate Regex Logic medium

If the regex is complex, simplify it or rethink the pattern to avoid unintended parenthesis usage.

1

Break down a complex regular expression into smaller, manageable parts. Test each part individually to identify where the imbalance occurs.

Instead of: `^((user|admin)_\d{4}|guest_[a-z]{3})$`
Test parts like: `^user_\d{4}$`, `^admin_\d{4}$`, `^guest_[a-z]{3}$`

2

Consider if the parentheses are necessary for the intended matching. Sometimes, simpler patterns can achieve the same result without grouping.

If you're using `(a|b)c` and `ac` or `bc` are the only possibilities, the parentheses might be redundant if the context doesn't require capturing or specific grouping.

3

Use online regex testers or debuggers to visualize and validate your regex. This can help pinpoint errors in parenthesis balancing.

Search for 'online regex tester' and input your pattern. Many tools highlight syntax errors.

4

Once the logic is clear and the regex is simplified, ensure all parentheses are correctly balanced.

SELECT * FROM your_table WHERE your_column REGEXP 'your_simplified_and_corrected_regex';

🔗

Related Errors

5 related errors

MySQL Error 3691: Mismatched Regex Parentheses