Bulk renaming makes it easy to accidentally create invalid or fragile filenames. Here's how to avoid that.
Windows reserves the following characters and won't allow them inside a filename:
| Character | Why it's blocked |
|---|---|
\ / : * ? " < > | | Reserved by the Windows file system for paths, wildcards, redirection, and other system uses. |
A small set of names are reserved by Windows regardless of file extension, and will cause errors if used as a filename:
CON, PRN, AUX, NUL
COM1–COM9, LPT1–LPT9
If a batch rename template could ever produce one of these exact names, add a distinguishing prefix or suffix rule to your rename stack to avoid collisions.
Very long combined folder-plus-filename paths can still cause problems in some older applications and on some networked drives, even though modern Windows versions support longer paths than in the past. When renaming files that will be stored deep inside nested folders, keep individual filenames reasonably concise.
If files will be shared with people on other operating systems or uploaded to services with strict validation, plain letters, numbers, hyphens, and underscores travel best.
Pick one convention — Title Case, snake_case, or kebab-case — and apply it uniformly with a single rename rule rather than mixing styles.
Use YYYY-MM-DD so files sort correctly by name in any file browser, instead of locale-dependent date formats.
Before applying a rename in bulk, check the preview for any two files that would end up with the identical new name — that's a sign your rule needs an added numbering step.
Check our compatibility and troubleshooting notes for common fixes.
Troubleshooting Guide