"Redundant character escape" in Kotlin Regular Expression

Question

I'm learning Kotlin and tried some regular expression things.

What I want to do is just remove all "comment" formats in the string (including \" in middle).
Like this: normal "commen\"t" to normal

So I wrote this code:

val comments = fileContents.replace(Regex("\"(.?(\\\")?)*?\""), "")

and I got: `Redundant character escape '\\\"' in RegExp" warning.

I tried IDE quick fix thing, and it changed '\\\"' to '\"' which is not a happy result for me.

Also, the same regular expression worked without any warning or error in Python. Here is the Python code:

re.sub(r'"(.?(\\")?)*?"', "", _f.read())

How can I remove this warning?

You need to escape both backslashes, use `\\\\\"` – mimak May 06 '23 at 09:46 — mimak, May 06 '23 at 09:46

Endzeit · Accepted Answer · 2023-05-06T11:12:30.517

tl;dr

Use raw Strings for creating Regex:

""""(.?(\\")?)*?"""".toRegex()

As you've written yourself, you need to escape some characters to actually get the regular expression you're looking for. Disregarding any special characters that need escaping, I assume you try to reach the following pattern: "(.?(\")?)*?".

To have an actual backslash character in your regular expression as literal character, you have to write four backslashes, like in Java. This is because the backslash is both an escape character for regular Strings as well as in Regexs.

The expression "\\" yields a string containing a single backslash. However, to get a literal backslash character in a regular expression, you have to escape it with another backslash character. That is:
The expression "\\\\" turns into a String containing of two '\' characters, that is "\\".
The String "\\" turned into a Regex becomes a regular expression containing a single backslash literal \.

You can see this more clearly by executing the following code:

println("\\\\")            // String with two backslash characters
println("\\\\".toRegex())  // Regex with single backslash literal

println("\\")              // String with single backslash characters
println("\\".toRegex())    // Exception in thread "main" java.util.regex.PatternSyntaxException: Unexpected internal error near index 1 \

In general I'd recommend to use raw strings, whenever creating a regular expression in Kotlin. They're delimited by triple quotes (""") instead of single quotes ("). In raw string literals, the backslash character is not an escape character, thus you neither have to escape it (for the String) nor the single quotes.

Instead of "\"(.?(\\\\\")?)*?\"" you can write """"(.?(\\")?)*?"""".

Additionally, you can use the extension function fun String.toRegex(): Regex, to convert your String to a Regex object, but that's just a question of preference.

All in all, your code could look like:

val fileContents = """normal "commen\"t" end"""

val regex = """"(.?(\\")?)*?"""".toRegex()
val comments = fileContents.replace(regex, "")

println(comments)   // prints "normal  end"

"Redundant character escape" in Kotlin Regular Expression

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