Oracle REGEXP and Newlines:
Why You Must Use the 'n' Flag with CHR(10)

Introduction

When working with Oracle REGEXP functions such as REGEXP_SUBSTR, REGEXP_INSTR, or REGEXP_REPLACE, handling newline characters (CHR(10) or CHR(13)) can introduce subtle and hard-to-diagnose issues. A common assumption is that regular expressions behave consistently regardless of line breaks. However, in Oracle, the presence of newline characters changes how the REGEXP engine evaluates patterns — especially when delimiters themselves include newlines. This article highlights a lesser-known but critical detail: the necessity of using the ‘n’ match parameter when working with REGEXP patterns that involve newline characters. Through a real-world example, we demonstrate how omitting this flag leads to incorrect matches, unexpected split results, and misleading position calculations — and how a single flag resolves all of them.

Problem Description

In real-world Oracle systems, it is common to store large text values that span multiple lines and require logical splitting into rows. In some scenarios, the delimiter itself is not a simple character but a compound pattern, such as a newline followed by another symbol.In this case, the requirement is to split a long string using CHR(10) || ‘/’ as the delimiter — meaning that a split should occur only when a newline is immediately followed by a slash.The input string follows this structure:

aaaaa/bbbbb
ccccc
/ddddd
eeeee
/ffffff

In SQL form:

'aaaaa/bbbbb' || CHR(10) || 'ccccc' || CHR(10) || '/ddddd' || CHR(10) || 'eeeee' || CHR(10) || '/ffffff'

Based on the delimiter definition, the expected output is:

aaaaa/bbbbb
ccccc
/ddddd
eeeee
/ffffff

However, when using REGEXP_SUBSTR without special match parameters, Oracle produces unexpected and incorrect results. Isolated newline characters interfere with pattern matching, causing segments to be skipped or split incorrectly. In more complex cases, related functions such as REGEXP_INSTR may also return inconsistent positions.

This behavior is particularly misleading because the regular expression itself appears logically correct, yet the output does not reflect the intended delimiter logic.

The Failing Implementation (Without the ‘n’ Flag)

The following implementation looks correct at first glance and is commonly used in production code.

However, when the delimiter includes a newline character, the results become incorrect and misleading.

WITH data AS (
   SELECT 'aaaaa/bbbbb' || CHR(10) || 'ccccc' || CHR(10) || '/ddddd' || CHR(10) || 'eeeee' || CHR(10) || '/ffffff' AS str,
          CHR(10)||'/' AS delim
      FROM DUAL
)
SELECT ROWNUM AS rrow,
       REGEXP_SUBSTR(
            str || delim,
            '(.*?)(' || delim || '|$)',
            1,
            LEVEL,
            NULL,
            1
      ) AS value
   FROM data
   CONNECT BY LEVEL <= REGEXP_COUNT(str, delim, 1, 'n') + 1;

Despite matching the intended delimiter, the output is incorrect. Isolated newline characters cause premature splits, and functions such as REGEXP_INSTR may return inconsistent positions.

Root Cause: Oracle REGEXP and Newline Handling

The root cause of this issue lies in how Oracle’s regular expression engine handles newline characters by default.In Oracle REGEXP functions, the dot (.) metacharacter does not match newline characters (CHR(10) and CHR(13)) unless explicitly instructed to do so. This behavior is compliant with traditional POSIX-style regular expressions, but it becomes problematic when:

The input string contains embedded newlines
The delimiter itself includes a newline character
The pattern relies on `.*?` or similar constructs to capture variable-length content

In the earlier example, the pattern: (.*?)(\n/|$)

appears logically correct. However, because . stops matching at newline boundaries, Oracle’s REGEXP engine internally breaks the match flow. As a result:

Partial segments are skipped
Match boundaries shift unexpectedly
Functions like REGEXP_INSTR may return non-linear or misleading positions

This behavior becomes even more confusing when isolated newline characters exist in the data that are not part of the delimiter. Oracle treats them as hard stop points for the dot operator, effectively fragmenting the input in ways that are not immediately visible from the pattern itself.

Crucially, this is not a bug in Oracle, nor an error in the regular expression syntax. It is a direct consequence of the default newline handling rules of the REGEXP engine.

The Correct Solution: Using the ‘n’ Flag

The correct and production-safe solution is to enable the ‘n’ (dotall) flag in Oracle REGEXP functions.

When the ‘n’ flag is specified, Oracle changes the behavior of the dot (.) metacharacter so that it matches newline characters (CHR(10) and CHR(13)) in addition to all other characters. This allows the REGEXP engine to treat the input string as a continuous stream rather than breaking matches at newline boundaries.

By applying this flag, patterns that rely on constructs such as `.*` or `.*?` behave consistently—even when:

The delimiter itself contains a newline
The input includes unrelated or isolated newline characters
The data spans multiple logical lines

Revised and correct query:

WITH data AS (
   SELECT 'aaaaa/bbbbb' || CHR(10) || 'ccccc' || CHR(10) || '/ddddd' || CHR(10) || 'eeeee' || CHR(10) || '/ffffff' AS str,
          CHR(10)||'/' AS delim
      FROM DUAL
)
SELECT ROWNUM AS rrow,
       REGEXP_SUBSTR(
            str || delim,
            '(.*?)(' || delim || '|$)',
            1,
            LEVEL,
            'n',
            1
      ) AS value
   FROM data
   CONNECT BY LEVEL <= REGEXP_COUNT(str, delim, 1, 'n') + 1;

With the ‘n’ flag enabled, the REGEXP engine correctly processes newline characters, and the delimiter is matched only when the exact sequence CHR(10)||’/’ occurs, producing the expected output without side effects.

Key Takeaways & Best Practices

When working with Oracle REGEXP functions, newline characters (CHR(10) and CHR(13)) introduce a subtle but critical behavior change that can easily break otherwise correct logic.

Keep the following rules in mind:

Always use the ‘n’ flag when your data or delimiter contains newline characters. Without it, the dot (.) metacharacter does not match newlines, leading to incomplete matches, incorrect splitting, and misleading results.
Do not confuse ‘n’ with ‘m’. The ‘m’ (multiline) flag only affects how `^` and `$` behave. It does not allow `.` to match newline characters and therefore does not solve this problem.
Apply the flag consistently across all REGEXP functions. If you use ‘n’ in REGEXP_SUBSTR, you should also use it in REGEXP_COUNT, REGEXP_INSTR, or REGEXP_REPLACE to avoid logical mismatches.
Expect hidden bugs when working with multi-line CLOBs. Logs, XML, JSON, generated text, and message payloads frequently contain newlines that are not visually obvious but directly affect REGEXP behavior.
This is not version-specific. The behavior exists from Oracle 11g through 23ai, so legacy systems are equally affected.

Practical Rule to Remember

If your REGEXP pattern touches a newline — directly or indirectly —
the ‘n’ flag is not optional. It is mandatory.

Understanding this rule can prevent hours of debugging and eliminate hard-to-diagnose production issues in PL/SQL and SQL code.

Oracle REGEXP and Newlines

Oracle REGEXP and Newlines:Why You Must Use the 'n' Flag with CHR(10)

Leave a Reply Cancel reply

Oracle REGEXP and Newlines:
Why You Must Use the 'n' Flag with CHR(10)