Windows Filename Validation Regex (Invalid Characters)

Validate Windows filenames by ensuring they don't contain invalid characters using a negated character class regex pattern.

Pattern Breakdown

[^\\/:*?"<>|]+

Components

Character Classes

• [^\\/:*?"<>|] - Negated character class: matches any character EXCEPT the listed invalid ones
• + - Quantifier: one or more occurrences

Examples

Valid:

• myfile.txt
• document (1).pdf
• file-name_123.docx
• test file.txt
• file.name.txt

Invalid:

• file/name.txt (contains forward slash)
• file:name.txt (contains colon)
• file*name.txt (contains asterisk)
• file?name.txt (contains question mark)
• file<name>.txt (contains angle brackets)
• file|name.txt (contains pipe)
• file\name.txt (contains backslash)
• file"name.txt (contains double quote)

Implementation

JavaScript

const filenameRegex = /[^\\/:*?"<>|]+/;
filenameRegex.test('myfile.txt'); // true
filenameRegex.test('document (1).pdf'); // true
filenameRegex.test('file/name.txt'); // false (contains invalid char)
filenameRegex.test('file:name.txt'); // false (contains colon)

// For full filename validation (with anchors)
const fullFilenameRegex = /^[^\\/:*?"<>|]+$/;
fullFilenameRegex.test('myfile.txt'); // true
fullFilenameRegex.test('file/name.txt'); // false

Python

import re
filename_regex = r'[^\\/:*?"<>|]+'
bool(re.match(filename_regex, 'myfile.txt'))  # True
bool(re.match(filename_regex, 'document (1).pdf'))  # True
bool(re.match(filename_regex, 'file/name.txt'))  # False (contains invalid char)

# For full filename validation
full_filename_regex = r'^[^\\/:*?"<>|]+$'
bool(re.match(full_filename_regex, 'myfile.txt'))  # True

Go

filenameRegex := regexp.MustCompile(`[^\\/:*?"<>|]+`)
filenameRegex.MatchString("myfile.txt") // true
filenameRegex.MatchString("document (1).pdf") // true
filenameRegex.MatchString("file/name.txt") // false (contains invalid char)

// For full filename validation
fullFilenameRegex := regexp.MustCompile(`^[^\\/:*?"<>|]+$`)
fullFilenameRegex.MatchString("myfile.txt") // true

Limitations

1. No reserved names check: Doesn't prevent reserved names like CON, PRN, AUX, NUL, etc.
2. No length limit: Doesn't enforce Windows filename length limits (260 chars for full path)
3. No trailing spaces/dots: Doesn't prevent trailing spaces or dots (invalid in Windows)
4. Pattern only: This pattern matches valid characters, not full filename validation
5. No path validation: Only validates filename part, not full path

When to Use

• Filename sanitization
• File upload validation
• Preventing invalid characters in filenames
• When you need to filter out Windows-invalid characters
• Input validation for file operations

For production, consider:

• Adding anchors (^...$) for full filename validation
• Checking for reserved Windows filenames (CON, PRN, AUX, NUL, COM1-9, LPT1-9)
• Preventing trailing spaces and dots
• Enforcing length limits (260 chars for full path, 255 for filename)
• Using OS-specific filename validation libraries
• Combining with path validation if needed