What is the difference between UCS-2 and UTF-16?
UCS-2 is what a Unicode implementation was up to Unicode 1.1, before surrogate code points and UTF-16 were added as concepts to Version 2.0 of the standard. This term should be now be avoided. When interpreting what people have meant by “UCS-2” in past usage, it is best thought of as not a data format, but as an indication that an implementation does not interpret any supplementary characters. In particular, for the purposes of data exchange, UCS-2 and UTF-16 are identical formats. Both are 16-bit, and have exactly the same code unit representation. The effective difference between UCS-2 and UTF-16 lies at a different level, when one is interpreting a sequence code units as code points or as characters. In that case, a UCS-2 implementation would not handle processing like character properties, codepoint boundaries, collation, etc. for supplementary characters.
UCS-2 corresponds to a Unicode implementation up to Unicode 1.1, before surrogate code points and UTF-16 were added to Version 2.0 of the standard. This term should now be avoided. UCS-2 does not define a data format, because UTF-16 and UCS-2 are identical for purposes of data exchange. Both are 16-bit, and have exactly the same code unit representation. Instead, “UCS-2” has sometimes been used in the past to indicate that an implementation does not support supplementary characters and doesn’t interpret pairs of surrogate code points as characters. Such an implementation would not handle processing like character properties, codepoint boundaries, collation, etc. for supplementary characters.
