Convert a Unicode string to an escaped ASCII string

You need to use the Convert() method in the Encoding class:

• Create an Encoding object that represents ASCII encoding
• Create an Encoding object that represents Unicode encoding
• Call Encoding.Convert() with the source encoding, the destination encoding, and the string to be encoded

There is an example here:

using System;
using System.Text;


namespace ConvertExample
{
class ConvertExampleClass
{
static void Main()
{
string unicodeString = "This string contains the unicode character Pi(\u03a0)";


// Create two different encodings.
Encoding ascii = Encoding.ASCII;
Encoding unicode = Encoding.Unicode;


// Convert the string into a byte[].
byte[] unicodeBytes = unicode.GetBytes(unicodeString);


// Perform the conversion from one encoding to the other.
byte[] asciiBytes = Encoding.Convert(unicode, ascii, unicodeBytes);


// Convert the new byte[] into a char[] and then into a string.
// This is a slightly different approach to converting to illustrate
// the use of GetCharCount/GetChars.
char[] asciiChars = new char[ascii.GetCharCount(asciiBytes, 0, asciiBytes.Length)];
ascii.GetChars(asciiBytes, 0, asciiBytes.Length, asciiChars, 0);
string asciiString = new string(asciiChars);


// Display the strings created before and after the conversion.
Console.WriteLine("Original string: {0}", unicodeString);
Console.WriteLine("Ascii converted string: {0}", asciiString);
}
}
}

string StringFold(string input, Func<char, string> proc)
{
return string.Concat(input.Select(proc).ToArray());
}


string FoldProc(char input)
{
if (input >= 128)
{
return string.Format(@"\u{0:x4}", (int)input);
}
return input.ToString();
}


string EscapeToAscii(string input)
{
return StringFold(input, FoldProc);
}

This goes back and forth to and from the \uXXXX format.

class Program {
static void Main( string[] args ) {
string unicodeString = "This function contains a unicode character pi (\u03a0)";


Console.WriteLine( unicodeString );


string encoded = EncodeNonAsciiCharacters(unicodeString);
Console.WriteLine( encoded );


string decoded = DecodeEncodedNonAsciiCharacters( encoded );
Console.WriteLine( decoded );
}


static string EncodeNonAsciiCharacters( string value ) {
StringBuilder sb = new StringBuilder();
foreach( char c in value ) {
if( c > 127 ) {
// This character is too big for ASCII
string encodedValue = "\\u" + ((int) c).ToString( "x4" );
sb.Append( encodedValue );
}
else {
sb.Append( c );
}
}
return sb.ToString();
}


static string DecodeEncodedNonAsciiCharacters( string value ) {
return Regex.Replace(
value,
@"\\u(?<Value>[a-zA-Z0-9]{4})",
m => {
return ((char) int.Parse( m.Groups["Value"].Value, NumberStyles.HexNumber )).ToString();
} );
}
}

Outputs:

This function contains a unicode character pi (π)

This function contains a unicode character pi (\u03a0)

This function contains a unicode character pi (π)

class Program
{
static void Main(string[] args)
{
char[] originalString = "This string contains the unicode character Pi(π)".ToCharArray();
StringBuilder asAscii = new StringBuilder(); // store final ascii string and Unicode points
foreach (char c in originalString)
{
// test if char is ascii, otherwise convert to Unicode Code Point
int cint = Convert.ToInt32(c);
if (cint <= 127 && cint >= 0)
asAscii.Append(c);
else
asAscii.Append(String.Format("\\u{0:x4} ", cint).Trim());
}
Console.WriteLine("Final string: {0}", asAscii);
Console.ReadKey();
}
}

All non-ASCII chars are converted to their Unicode Code Point representation and appended to the final string.

To store actual Unicode codepoints, you have to first decode the String's UTF-16 codeunits to UTF-32 codeunits (which are currently the same as the Unicode codepoints). Use System.Text.Encoding.UTF32.GetBytes() for that, and then write the resulting bytes to the StringBuilder as needed,i.e.

static void Main(string[] args)
{
String originalString = "This string contains the unicode character Pi(π)";
Byte[] bytes = Encoding.UTF32.GetBytes(originalString);
StringBuilder asAscii = new StringBuilder();
for (int idx = 0; idx < bytes.Length; idx += 4)
{
uint codepoint = BitConverter.ToUInt32(bytes, idx);
if (codepoint <= 127)
asAscii.Append(Convert.ToChar(codepoint));
else
asAscii.AppendFormat("\\u{0:x4}", codepoint);
}
Console.WriteLine("Final string: {0}", asAscii);
Console.ReadKey();
}

A small patch to @Adam Sills's answer which solves FormatException on cases where the input string like "c:\u00ab\otherdirectory\" plus RegexOptions.Compiled makes the Regex compilation much faster:

    private static Regex DECODING_REGEX = new Regex(@"\\u(?<Value>[a-fA-F0-9]{4})", RegexOptions.Compiled);
private const string PLACEHOLDER = @"#!#";
public static string DecodeEncodedNonAsciiCharacters(this string value)
{
return DECODING_REGEX.Replace(
value.Replace(@"\\", PLACEHOLDER),
m => {
return ((char)int.Parse(m.Groups["Value"].Value, NumberStyles.HexNumber)).ToString(); })
.Replace(PLACEHOLDER, @"\\");
}

As a one-liner:

var result = Regex.Replace(input, @"[^\x00-\x7F]", c =>
string.Format(@"\u{0:x4}", (int)c.Value[0]));

For Unescape You can simply use this functions:

System.Text.RegularExpressions.Regex.Unescape(string)


System.Uri.UnescapeDataString(string)

I suggest using this method (It works better with UTF-8):

UnescapeDataString(string)

Here is my current implementation:

public static class UnicodeStringExtensions
{
public static string EncodeNonAsciiCharacters(this string value) {
var bytes = Encoding.Unicode.GetBytes(value);
var sb = StringBuilderCache.Acquire(value.Length);
bool encodedsomething = false;
for (int i = 0; i < bytes.Length; i += 2) {
var c = BitConverter.ToUInt16(bytes, i);
if ((c >= 0x20 && c <= 0x7f) || c == 0x0A || c == 0x0D) {
sb.Append((char) c);
} else {
sb.Append($"\\u{c:x4}");
encodedsomething = true;
}
}
if (!encodedsomething) {
StringBuilderCache.Release(sb);
return value;
}
return StringBuilderCache.GetStringAndRelease(sb);
}




public static string DecodeEncodedNonAsciiCharacters(this string value)
=> Regex.Replace(value,/*language=regexp*/@"(?:\\u[a-fA-F0-9]{4})+", Decode);


static readonly string[] Splitsequence = new [] { "\\u" };
private static string Decode(Match m) {
var bytes = m.Value.Split(Splitsequence, StringSplitOptions.RemoveEmptyEntries)
.Select(s => ushort.Parse(s, NumberStyles.HexNumber)).SelectMany(BitConverter.GetBytes).ToArray();
return Encoding.Unicode.GetString(bytes);
}
}

This passes a test:

public void TestBigUnicode() {
var s = "\U00020000";
var encoded = s.EncodeNonAsciiCharacters();
var decoded = encoded.DecodeEncodedNonAsciiCharacters();
Assert.Equals(s, decoded);
}

with the encoded value: "\ud840\udc00"

This implementation makes use of a StringBuilderCache (reference source link)