Ignore existing spaces in converting CamelCase to string with spaces

Question

I want to split camelCase or PascalCase words to space separate collection of words.

So far, I have:

Regex.Replace(value, @"(\B[A-Z]+?(?=[A-Z][^A-Z])|\B[A-Z]+?(?=[^A-Z]))", " $0", RegexOptions.Compiled);

It works fine for converting "TestWord" to "Test Word" and for leaving single words untouched, e.g. Testing remains Testing.

However, ABCTest gets converted to A B C Test when I would prefer ABC Test.

Answer 1

Try:

[A-Z][a-z]+|[A-Z]+(?=[A-Z][a-z])|[a-z]+|[A-Z]+

An example on Regex101

---

How is it used in CS?

string strText = " TestWord asdfDasdf  ABCDef";
        
string[] matches = Regex.Matches(strText, @"[A-Z][a-z]+|[A-Z]+(?=[A-Z][a-z])|[a-z]+|[A-Z]+")
                .Cast<Match>()
                .Select(m => m.Value)
                .ToArray();
            
string result = String.Join(" ", matches);

result = 'Test Word asdf Dasdf ABC Def'

---

How it works

In the example string:

TestWord qwerDasdf
ABCTest Testing    ((*&^%$CamelCase!"£$%^^))
asdfAasdf
AaBbbCD

[A-Z][a-z]+ matches:

• [0-4] Test
• [4-8] Word
• [13-18] Dasdf
• [22-26] Test
• [27-34] Testing
• [45-50] Camel
• [50-54] Case
• [68-73] Aasdf
• [74-76] Aa
• [76-79] Bbb

[A-Z]+(?=[A-Z][a-z]) matches:

• [19-22] ABC

[a-z]+ matches:

• [9-13] qwer
• [64-68] asdf

[A-Z]+ matches:

• [79-81] CD

Answer 2

Here is my attempt:

(?<!^|\b|\p{Lu})\p{Lu}+(?=\p{Ll}|\b)|(?<!^\p{Lu}*|\b)\p{Lu}(?=\p{Ll}|(?<!\p{Lu}*)\b)

This regex can be used with Regex.Replace and $0 as a replacement string.

Regex.Replace(value, @"(?<!^|\b|\p{Lu})\p{Lu}+(?=\p{Ll}|\b)|(?<!^\p{Lu}*|\b)\p{Lu}(?=\p{Ll}|(?<!\p{Lu}*)\b)", " $0", RegexOptions.Compiled);

See demo

Regex Explanation:

• Contains 2 alternatives to account for a chain of capital letters before or after lowercase letters.
• (?<!^|\b|\p{Lu})\p{Lu}+(?=\p{Ll}|\b) - first alternative that matches several uppercase letters that are not preceded with a start of string, word boundary or another uppercase letter, and that are followed by a lowercase letter or a word boundary,
• (?<!^\p{Lu}*|\b)\p{Lu}(?=\p{Ll}|(?<!\p{Lu}*)\b) - the second alternative that matches a single capital letter that is not preceded with a start of string with optional uppercase letters right after, or word boundary and is followed by a lowercase letter or a word boundary that is not preceded by optional uppercase letters.