Illegal characters are symbols that violate the “law” of a computer program or syntax, and can vary depending on the program used. Some characters are banned in certain settings, and different operating systems have different rules. Some non-English letters and symbols are often illegal in English-language programs. Certain characters are also illegal for email addresses or URLs. System administrators may ban users who violate the terms of use by using prohibited characters.
Illegal characters are letters, numbers, punctuation marks, and similar symbols that are disallowed or prohibited by a computer program or syntax. The exact nature of these characters can vary, depending on the program you use and how those characters are used. This means that the context in which a person is working can often change which characters they can use and which ones can cause problems. Illegal characters in English are usually punctuation marks or letters that include unusual accents.
There are many different letters and symbols that may be considered illegal characters, and there are also other characters that may be banned even if not necessarily illegal in certain settings. The term “illegal” in this usage does not refer to any particular law, but simply indicates that the use of such characters violates the “law” of a particular computer program. For example, in the Windows® operating system (OS), symbols such as colons, square brackets, and question marks are usually illegal characters to use to name files or folders.
However, many of these illegal characters are allowed to name files and folders when using Mac OS®, because the programming behind these systems is different. A colon is an invalid character in both types of operating systems, as it is used in both systems to distinguish between files and folders in a file path. Different versions of these systems may also allow certain characters that are not allowed in other versions, so system administrators often need to be aware of different illegal characters that can create problems in different types of systems. Some letters that can be found in non-English languages, especially English letters that include accent marks quite common in French or Spanish, are often illegal characters in English-language programs.
There are also some characters that are illegal for particular types of systems, such as email addresses or URLs (Uniform Resource Locator). These systems often disallow certain characters or spaces in particular combinations or positions, such as three dots together in what would be an ellipsis in most text settings, or spaces at the beginning or end of a name. Some chat programs or services may also prohibit the use of certain letters that include accent marks, which are not necessarily illegal but are prohibited to keep the system easier to read and manage. System administrators can potentially ban users who routinely use these prohibited characters, as such use could violate the terms of use for that program or service.
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