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Description

This MySQL error indicates that a regular expression used in a query contains an escape sequence (`\character`) that the database's regex engine does not recognize or considers malformed. It commonly occurs when a backslash is used incorrectly or when a pattern intended for a different regex flavor is applied.

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Error Message

Unrecognized escape sequence in regular expression.

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Known Causes

4 known causes

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Incorrect Character Escaping

A backslash (`\`) is used before a character that does not form a valid escape sequence according to MySQL's regex rules, leading to an unrecognized pattern.

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Incompatible Regex Flavor

The regular expression contains escape sequences or syntax conventions that are specific to a different regex engine (e.g., PCRE, JavaScript) and not recognized by MySQL's implementation.

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Typographical Error or Incomplete Sequence

A mistake in typing an escape sequence (e.g., `\d` instead of `\s`) or an incomplete sequence like a lone backslash at the end of the pattern.

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SQL String Literal Escaping Conflict

Backslashes are incorrectly handled by both the SQL string parser and the regex engine, causing an unintended or malformed sequence to be passed to the regex engine.

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Solutions

3 solutions available

1. Escape Backslashes in String Literals easy

Ensure backslashes within string literals used in REGEXP are properly escaped.

1

Identify the regular expression pattern in your SQL query.

2

If the pattern contains a backslash (`\`), it needs to be escaped by another backslash within the SQL string literal. For example, to match a literal backslash, use `'\\'` in your SQL.

SELECT 'some_string' REGEXP '\\'; -- Matches a literal backslash

3

If you are using a backslash followed by a character that has special meaning in regular expressions (e.g., `\d`, `\s`, `\w`), ensure both the backslash and the special character are correctly escaped. MySQL's REGEXP operator often interprets `\` as a literal backslash within the pattern itself, so you might need to double-escape if the character following the backslash is also an escape character in SQL strings.

SELECT 'abc 123' REGEXP '\\d'; -- Matches a digit (literal \d in regex)

2. Use Raw String Literals (if supported by client/application) medium

Leverage raw string literals in your application code to avoid double-escaping.

1

If you are constructing SQL queries from an application (e.g., Python, Java, PHP), check if your programming language supports raw string literals. These literals treat backslashes as literal characters, simplifying the construction of regex patterns.

2

In Python, you can use `r'...'` for raw strings. For example, to match a literal backslash, you would use `r'\'` in your Python code, which directly translates to the SQL pattern `\`.

import mysql.connector

conn = mysql.connector.connect(...)
c = conn.cursor()

regex_pattern = r'\\'
query = "SELECT * FROM my_table WHERE my_column REGEXP %s"
c.execute(query, (regex_pattern,))

3

In other languages, similar constructs might exist. This approach moves the burden of escaping from the SQL query itself to the application code, making the regex pattern more readable and less prone to errors.

3. Simplify or Rephrase the Regular Expression medium

Reconstruct the regex pattern to avoid ambiguous or unsupported escape sequences.

1

Examine the regular expression for any unusual or non-standard escape sequences. MySQL's REGEXP implementation is based on POSIX extended regular expressions, but there can be nuances with backslash escapes.

2

If you are trying to match a literal character that has a special meaning in regex (e.g., `.`, `*`, `+`, `?`, `^`, `$`, `(`, `)`, `[`, `]`, `{`, `}`, `|`, `\`), always escape it with a backslash. Ensure the backslash itself is correctly escaped in the SQL string.

SELECT 'file.txt' REGEXP 'file\\.txt'; -- Matches 'file.txt' (escaped dot)

3

Consider using character classes or alternative syntax if a specific escape sequence is causing issues. For example, instead of a potentially problematic escape for whitespace, you might use `[[:space:]]`.

SELECT 'line with spaces' REGEXP '[[:space:]]'; -- Matches any whitespace character

4

If you are unsure about a specific escape sequence, consult the MySQL documentation for the `REGEXP` operator and POSIX regular expression syntax.

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Related Errors

5 related errors

MySQL Error 3689: Invalid Regular Expression Escape