Kotlin converting "literal escapes" to escaped characters

Question

I'm creating a parser and I need to convert strings with escaped characters to the characters themselves. Suppose I have a string "\n" (backslash and letter n), I want to have some algorithm to transform it to line break '\n'.

Straightforward way is to create a map where literal escaped strings will be matched with escaped characters, but this solution is bad if I want to convert unicode escapes (like, "\uFF00" to '\uFF00').

Is there any other way to solve this rather than creating a map?

Answer 1

You can do something like the following.

1. Write a function that iterates over the string character by character. If you encounter a \ character, then try to parse an escape sequence beginning with that.

data class CharacterLiteral(
    val value: Char?,
    val length: Int,
)

fun String.format(): String {
    val builder = StringBuilder()

    var i = 0
    while (i < this.length) {
        val ch = this[i]
        if (ch == '\\') {
            val characterLiteral = parseEscapedCharacter(i + 1)
            if (characterLiteral.value == null) {
                i++
            } else {
                builder.append(characterLiteral.value)
                i += characterLiteral.length
            }
        } else {
            builder.append(ch)
            i++
        }
    }

    return builder.toString()
}

2. Parse the escaped character

fun String.parseEscapedCharacter(index: Int): CharacterLiteral {
    return when (getOrNull(index)) {
        'u' -> parseUnicodeCharacter(index + 1)
        't' -> CharacterLiteral('\t', 2)
        'n' -> CharacterLiteral('\n', 2)
        '\'' -> CharacterLiteral('\'', 2)
        '"' -> CharacterLiteral('"', 2)
        '\\' -> CharacterLiteral('\\', 2)
        else -> CharacterLiteral(null, 0) // invalid case
    }
}

3. Parse the unicode character

fun String.parseUnicodeCharacter(index: Int): CharacterLiteral {
    // unicode: \u0000 to \uFFFF
    for (i in 0..<4) {
        val ch = getOrNull(index + i)
        if (ch == null || (!ch.isDigit() && ch.lowercase() !in "abcdef")) {
            return CharacterLiteral(null, 0) // invalid
        }
    }

    // Source: https://stackoverflow.com/a/45273638/8822610
    val unicodeNumber = this.slice(index..(index + 3)).trimStart('0')
    val string = Character.toString(Integer.parseInt(unicodeNumber, 16))
    return CharacterLiteral(string[0], 6)
}

You can compare the result of the format function.

fun main() {
    val s1 = "bla '\\u0065' '\\t' '\\n' '\\\\'"
    val s2 = "bla '\u0065' '\t' '\n' '\\'"
    println(s1.format() == s2) // true
}