What does [“string”].pack('H*') mean?

https://stackoverflow.com/questions/17623668/what-does-string-packh-mean

It interprets the string as hex numbers, two characters per byte, and converts it to a string with the characters with the corresponding ASCII code:

["464F4F"].pack('H*')
# =>
"FOO", 0x46 is the code for 'F', 0x4F the code for 'O'

把这个字符串当成是16进制，而且每两个字符当成是一个数，正好好一个字节，按照ASCII编码。

For the opposite conversion, use unpack:

'FOO'.unpack('H*')
# => ["464f4f"]

It is a little bit more difficult for non-ASCII-8BIT encodings:

"á".encoding
# => #<Encoding:UTF-8>
"á".unpack('H*')
# => ["c3a1"]
['c3a1'].pack('H*')
# => "xC3xA1"
['c3a1'].pack('H*').encoding
# => #<Encoding:ASCII-8BIT>
['c3a1'].pack('H*').force_encoding('UTF-8') # => "á"