Restricted characters in file names are symbols disallowed by operating systems to prevent conflicts with system operations or reserved functions. For instance, Windows prohibits characters like :, *, ", ?, <, >, |, /, and \ because they have special meanings in file paths, commands, or denote devices. Similarly, Unix-based systems (including Linux and macOS) forbid the forward slash ( / ), which separates directories in paths, and the null character. These rules differ across systems but commonly exclude symbols that would create ambiguity.

In practice, Windows blocks the use of colons (:) in file names locally but may permit them in cloud storage filenames. Uploading files containing slashes (/) to web servers often requires automatic renaming by the platform. Developers might face errors when generating log files named "data:report.txt" on Windows or attempting "/temp/log" on a Linux server.

These restrictions ensure system stability but pose challenges for cross-platform file sharing, leading to errors or corrupted transfers. Workarounds include automated character substitution during file uploads or using underscores/hyphens. As file systems evolve, some newer cloud platforms offer greater naming flexibility, though core system limitations persist for compatibility.