Regular expression to match a line that doesn't contain a word ?

To create a regular expression that matches a line not containing a specific word, use a negative lookahead assertion. This ensures the word does not appear anywhere in the line. Here’s how to do it:

Regex Pattern

^(?!.*\bword\b).*$

Explanation:

^: Start of the line.
(?!.*\bword\b): Negative lookahead to ensure the line does not contain the word word (with word boundaries).
.*: Match any characters (except newlines) until the end of the line.
$: End of the line.

Examples

1. Basic Usage

^(?!.*error).*$

Matches lines without the word error.
Example Match: "Success: Operation completed."
No Match: "Error: Invalid input."

2. Case-Insensitive Match

Add the i flag to ignore case:

^(?!.*error).*$  # Case-sensitive (default)
^(?i)(?!.*error).*$  # Case-insensitive (syntax varies by regex engine)

Example Match: "WARNING: No issues found." (avoids ERROR, Error, etc.).

Key Modifications

1. Exclude Empty Lines

Use .+ instead of .* to require at least one character:

^(?!.*error).+$

2. Match Partial Words (No Word Boundaries)

Remove \b to match substrings:

^(?!.*error).*$

Example No Match: "debuggerror" (contains “error” as a substring).

3. Multiline Input

Use the m (multiline) flag to match across multiple lines:

/^(?!.*error).*$/gm  # In JavaScript

Code Implementation

JavaScript

const regex = /^(?!.*\berror\b).*$/gm;
const text = `
Success: Task completed.
Error: File not found.
Warning: Low memory.
`;

// Filter lines without "error"
const validLines = text.match(regex) || [];
console.log(validLines); // ["Success: Task completed.", "Warning: Low memory."]

Python

import re

text = """
Success: Task completed.
Error: File not found.
Warning: Low memory.
"""

pattern = re.compile(r'^(?!.*\berror\b).*$', re.MULTILINE)
valid_lines = [line.strip() for line in pattern.findall(text)]
print(valid_lines)  # ['Success: Task completed.', 'Warning: Low memory.']

Common Edge Cases

Empty Lines:

^(?!.*error).*$ matches empty lines. Use ^(?!.*error).+$ to exclude them.

Special Characters:

Escape regex metacharacters (e.g., \bfoo\b becomes \bfoo\b if foo is literal).

Multi-Line Inputs:

Use flags like m (multiline) and s (dotall) depending on your regex engine.

Why This Works

The negative lookahead (?!.*word) checks the entire line for the absence of word.
Word boundaries (\b) prevent partial matches (e.g., err in error).

Use this regex to filter logs, validate input, or parse files while excluding lines with specific keywords!

Regular expression to match a line that doesn’t contain a word ?