Illegal characters are letters, numbers, punctuation marks, and similar symbols not allowed or prohibited by a computer program or syntax. The exact nature of these characters can vary, depending on the program being used and the way in which those characters are used. This means that the context in which a person is working can often change what characters he or she may use and which characters can create issues. Illegal characters in English are typically punctuation marks or letters that include unusual accent marks.
There are many different letters and symbols that can be considered illegal characters, and there are also other characters that might be prohibited though not necessarily illegal in certain settings. The term “illegal” in this usage does not refer to any particular laws, but instead simply indicates that the use of such characters violates the “law” of a particular computer program. In the Windows® operating system (OS), for example, symbols such as a colons, brackets, and question marks are typically illegal characters for use in naming files or folders.
Many of these illegal characters are allowed for naming files and folders when using the Mac OS®, however, since the programming behind these systems is different. The colon is an illegal character in both types of OS, as it is used in both systems to differentiate between files and folders in a file path. Different versions of these systems can also allow certain characters that are not allowed in other versions, so system administrators must often be aware of different illegal characters that can create issues in different types of systems. Certain letters that may be found in non-English languages, especially English letters that include accent marks that are quite common in French or Spanish, are often illegal characters in English language programs.