如何确定字符串是否被压缩？

Question

How can I determine whether a string was compressed with gzcompress (aparts from comparing sizes of string before/after calling gzuncompress, or would that be the proper way of doing it) ?

Answer 1

A string and a compressed string are both simply sequences of bytes. You cannot really distinguish one sequence of bytes from another sequence of bytes. You should know whether a blob of bytes represents a compressed format or not from accompanying metadata.

If you really need to guess programmatically, you have several things you can try:

• Try to uncompress the string and see if the uncompress operation succeeds. If it fails, the bytes probably did not represent a compressed string.
• Try to check for obvious "weird" bytes like anything before 0x20. Those bytes aren't typically used in regular text. There's no real guarantee that they occur in a compressed string though.
• Use mb_check_encoding to see whether a string is valid in the encoding you suspect it to be in. If it isn't, it's probably compressed (or you checked for the wrong encoding). With the caveat that virtually any byte sequence is valid in virtually every single-byte encoding, so this'll only work for multi-byte encodings.