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Description

The ORA-30186 error occurs when using the UNISTR function in Oracle Database. It indicates an invalid escape sequence within the UNISTR function's argument, specifically that a backslash is not followed by four hexadecimal characters or another backslash.

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Error Message

'' must be followed by four hexdecimal characters or another ''

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Known Causes

3 known causes

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Invalid Hex Sequence

A backslash in the UNISTR argument is followed by characters that are not four valid hexadecimal digits (0-9, A-F).

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Missing Escape Character

A single backslash is present where a double backslash (\\) is required to represent a literal backslash character.

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Typographical Error

There's a typo in the UNISTR string literal, causing an unintended backslash sequence.

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Solutions

3 solutions available

1. Correct UNISTR Escape Sequence Usage easy

Ensure escape sequences in UNISTR are properly formatted with four hex characters or escaped backslashes.

1

Review your `UNISTR` function calls. The escape character is typically a backslash (`\`).

2

If you intend to escape a literal backslash, use two backslashes (`\\`).

SELECT UNISTR('This is a literal backslash: \\') FROM dual;

3

If you are using a Unicode escape sequence, it must be followed by exactly four hexadecimal characters. For example, to represent the euro symbol (€), use `\20AC`.

SELECT UNISTR('The euro symbol is: \20AC') FROM dual;

4

Ensure there are no single backslashes followed by non-hexadecimal characters or less than four hexadecimal characters when a Unicode escape is intended. The error message indicates a malformed escape sequence.

2. Validate Input Data for UNISTR medium

Sanitize or validate any external input being passed to the UNISTR function to prevent malformed escape sequences.

1

If the string passed to `UNISTR` originates from user input, application logic, or external files, implement validation.

2

Before calling `UNISTR`, check if the string contains a lone backslash that is not part of a valid Unicode escape sequence (e.g., `\XXXX` where X is a hex digit) or an escaped backslash (`\\`).

3

Consider replacing or escaping problematic backslashes in the input string. For instance, if a lone backslash is not intended as an escape, you might replace it with `\\` if it's meant to be a literal backslash, or remove it if it's erroneous.

DECLARE
  input_string VARCHAR2(100) := 'Invalid \ escape';
  corrected_string VARCHAR2(100);
BEGIN
  -- Example: Replace lone backslashes intended as literal backslashes
  corrected_string := REPLACE(input_string, '\', '\\');
  DBMS_OUTPUT.PUT_LINE(UNISTR(corrected_string));
END;
/

3. Understand UNISTR Escape Character Behavior easy

Clarify how UNISTR interprets backslashes and ensure your strings adhere to its parsing rules.

1

The `UNISTR` function uses the backslash (`\`) as the escape character. This means a backslash is used to signal that the character(s) following it have a special meaning.

2

Two backslashes (`\\`) are interpreted as a single literal backslash character.

SELECT UNISTR('\\') FROM dual; -- Output: \

3

A backslash followed by four hexadecimal characters (`\XXXX`) is interpreted as the Unicode character represented by those hex digits.

SELECT UNISTR('\0041') FROM dual; -- Output: A

4

The error `ORA-30186` specifically occurs when `UNISTR` encounters a single backslash (`\`) that is not followed by four hexadecimal characters or another backslash. This indicates an incomplete or malformed escape sequence.

Oracle UNISTR Escape Sequence