• Notational Compatibility with Other Regex Flavors

    Note that a regex like [a-z-[aeiuo]] does not cause any errors in most regex flavors that do not support character class subtraction. But it won't match what you intended either. In most flavors, this regex consists of a character class followed by a literal ]. The character class matches a character that is either in the range a-z, or a hyphen, or an opening bracket, or a vowel. Since the a-z range and the vowels are redundant, you could write this character class as [a-z-[] or [-[a-z] in Perl. A hyphen after a range is treated as a literal character, just like a hyphen immediately after the opening bracket. This is true in the XML, .NET and JGsoft flavors too. [a-z-_]matches a lowercase letter, a hyphen or an underscore in these flavors.
    Strictly speaking, this means that the character class subtraction syntax is incompatible with Perl and the majority of other regex flavors. But in practice there's no difference. Using non-alphanumeric characters in character class ranges is very bad practice because it relies on the order of characters in the ASCII character table. That makes the regular expression hard to understand for the programmer who inherits your work. While [A-[] would match any upper case letter or an opening square bracket in Perl, this regex is much clearer when written as [A-Z[]. The former regex would cause an error with the XML, .NET and JGsoft flavors, because they interpret -[] as an empty subtracted class, leaving an unbalanced [.
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