Automatically escape unicode characters

After digging into some documentation about iconv, I think you can accomplish this using only the base package. But you need to pay extra attention to the encoding of the string.

On a system with UTF-8 encoding:

> stri_escape_unicode("你好世界")
[1] "\\u4f60\\u597d\\u4e16\\u754c"

# use big endian
> iconv(x, "UTF-8", "UTF-16BE", toRaw=T)
[[1]]
[1] 4f 60 59 7d 4e 16 75 4c

> x <- "•"
> iconv(x, "UTF-8", "UTF-16BE", toRaw=T)    
[[1]]
[1] 20 22

But, if you are on a system with latin1 encoding, things may go wrong.

> x <- "•"
> y <- "\u2022"
> identical(x, y)
[1] FALSE
> stri_escape_unicode(x)
[1] "\\u0095" # <- oops!

# culprit
> Encoding(x)
[1] "latin1"

# and it causes problem for iconv
> iconv(x, Encoding(x), "Unicode")
Error in iconv(x, Encoding(x), "Unicode") : 
  unsupported conversion from 'latin1' to 'Unicode' in codepage 1252
> iconv(x, Encoding(x), "UTF-16BE")
Error in iconv(x, Encoding(x), "UTF-16BE") : 
  embedded nul in string: '\0•'

It is safer to cast the string into UTF-8 before converting to Unicode:

> iconv(enc2utf8(enc2native(x)), "UTF-8", "UTF-16BE", toRaw=T)
[[1]]
[1] 20 22

EDIT: This may cause some problems for strings already in UTF-8 encoding on some particular systems. Maybe it's safer to check the encoding before conversion.

> Encoding("•")
[1] "latin1"
> enc2native("•")
[1] "•"
> enc2native("\u2022")
[1] "•"
# on a Windows with default latin1 encoding
> Encoding("测试") 
[1] "UTF-8"
> enc2native("测试") 
[1] "<U+6D4B><U+8BD5>"   # <- BAD! 

For some characters or lanuages, UTF-16 may not be enough. So probably you should be using UTF-32 since

The UTF-32 form of a character is a direct representation of its codepoint.

Based on above trial and error, below is probably one safer escape function we can write:

unicode_escape <- function(x, endian="big") {
  if (Encoding(x) != 'UTF-8') {
    x <- enc2utf8(enc2native(x))
  }
  to.enc <- ifelse(endian == 'big', 'UTF-32BE', 'UTF-32LE')

  bytes <- strtoi(unlist(iconv(x, "UTF-8", "UTF-32BE", toRaw=T)), base=16)
  # there may be some better way to do thibs.
  runes <- matrix(bytes, nrow=4)
  escaped <- apply(runes, 2, function(rb) {
    nonzero.bytes <- rb[rb > 0]
    ifelse(length(nonzero.bytes) > 1, 
           # convert back to hex
           paste("\\u", paste(as.hexmode(nonzero.bytes), collapse=""), sep=""),
           rawToChar(as.raw(nonzero.bytes))
           )
  })
  paste(escaped, collapse="")
}

Tests:

> unicode_escape("•••ERROR!!!•••")
[1] "\\u2022\\u2022\\u2022ERROR!!!\\u2022\\u2022\\u2022"
> unicode_escape("Hello word! 你好世界！")
[1] "Hello word! \\u4f60\\u597d\\u4e16\\u754c!"
> "\u4f60\u597d\u4e16\u754c"
[1] "你好世界"