US-ASCII string support.
References.
is_valid s is true iff only for all indices i of s, s.[i] is an US-ASCII character, i.e. a byte in the range [0x00;0x1F].
The following functions act only on US-ASCII code points that is on bytes in range [0x00;0x7F], leaving any other byte intact. The functions can be safely used on UTF-8 encoded strings; they will of course only deal with US-ASCII casings.
uppercase s is s with US-ASCII characters 'a' to 'z' mapped to 'A' to 'Z'.
lowercase s is s with US-ASCII characters 'A' to 'Z' mapped to 'a' to 'z'.
capitalize s is like uppercase but performs the map only on s.[0].
uncapitalize s is like lowercase but performs the map only on s.[0].
escape s is s with:
'\\' (0x5C) escaped to the sequence "\\\\" (0x5C,0x5C).0x00;0x1F] and [0x7F;0xFF] escaped by an hexadecimal "\xHH" escape with H a capital hexadecimal number. These bytes are the US-ASCII control characters and non US-ASCII bytes.unescape s unescapes what escape did. The letters of hex escapes can be upper, lower or mixed case, and any two letter hex escape is decoded to its corresponding byte. Any other escape not defined by escape or truncated escape makes the function return None.
The invariant unescape (escape s) = Some s holds.
escape_string s is like escape except it escapes s according to OCaml's lexical conventions for strings with:
'\b' (0x08) escaped to the sequence "\\b" (0x5C,0x62).'\t' (0x09) escaped to the sequence "\\t" (0x5C,0x74).'\n' (0x0A) escaped to the sequence "\\n" (0x5C,0x6E).'\r' (0x0D) escaped to the sequence "\\r" (0x5C,0x72).'\"' (0x22) escaped to the sequence "\\\"" (0x5C,0x22).escapeunescape_string is to escape_string what unescape is to escape and also additionally unescapes the sequence "\\'" (0x5C,0x27) to "'" (0x27).