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Is there some simpler way than the one below to convert a CamelCased string to a string consisting of separate words?

A crude way, with tedious typing, is the following:

unCamelCase[str_] := 
 StringDrop[
  StringJoin[
   StringSplit[
    str, {"A" -> " a", "B" -> " b", "C" -> " c", "H" -> " h", 
     "I" -> " i", "O" -> " o", "S" -> " s", "Z" -> " z"}]], 1]

(In the above I omitted from the list of replacements in StringSplit the letters I don't need for the particular example below, but for the method being used, in general I'd need the list of all 26 replacements.)

For example:

   unCamelCase["CompactSubspaceOfHausdorffSpaceIsClosed"]
(* compact subspace of hausdorff space is closed *)
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5 Answers 5

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There are two resource functions: FromCamelCase and ToCamelCase. FromCamelCase splits camel case phrases into words. Add ToLowerCase to convert to lower case.

ResourceFunction["FromCamelCase"]["CompactSubspaceOfHausdorffSpaceIsClosed"]
(* "Compact Subspace Of Hausdorff Space Is Closed" *)
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StringReplace["CompactSubspaceOfHausdorffSpaceIsClosed", 
 u_?UpperCaseQ :> " " <> ToLowerCase[u]]

(*" compact subspace of hausdorff space is closed"*)
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    $\begingroup$ I like your answer. It does create an extra space at the beginning so perhaps adding StringTrim would help. Although that would remove spaces if they existed. $\endgroup$
    – Mark R
    Sep 2, 2021 at 2:08
  • $\begingroup$ @MarkR That was not the original question. However that can be addressed by adding one more rule StringReplace["CompactSub...", {RegularExpression["^\\s*[A-Z]"] :> ToLowerCase["$0"], u_?UpperCaseQ :> " " <> ToLowerCase[u]}]. It depends what exactly is required. $\endgroup$
    – Suba Thomas
    Sep 2, 2021 at 13:41
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This handles acronyms and numbers properly:

deCamelCase[s_]:=StringRiffle@StringSplit[s,{RegularExpression["(?<=[A-Z])(?=[A-Z][a-z])"],RegularExpression["(?<=[^A-Z])(?=[A-Z])"],RegularExpression["(?<=[A-Za-z])(?=[^A-Za-z])"]}]

Based on an answer from https://stackoverflow.com/questions/2559759/how-do-i-convert-camelcase-into-human-readable-names-in-java

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splitCamelCase = ToLowerCase @ 
   StringInsert[#, " ", StringPosition[#, _?UpperCaseQ][[2;;, 1]]] &;

splitCamelCase @ "CompactSubspaceOfHausdorffSpaceIsClosed"

"compact subspace of hausdorff space is closed"
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  • $\begingroup$ This won't work correctly with acronyms inside of the string. But has the advantage of supporting languages other than English. $\endgroup$
    – Alexey Popkov
    Sep 2, 2021 at 10:49
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    $\begingroup$ Of course the proper noun "Hausdorff" should not be lower-cased, but that's a semantic issue rather than a syntactical issue that could only be addressed by some kind of dictionary look-up to identify proper nouns (and even that can be ambiguous). $\endgroup$
    – murray
    Sep 3, 2021 at 13:39
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Use a Regular Expression with a positive look-behind to insert a space before each capital letter that is preceded by either a lower-case letter or a digit (that is, insert a space before each capital letter except the first)

StringReplace[str, RegularExpression["(?<=[a-z1-9])([A-Z])"]:> " $1" ] // ToLowerCase

    (*compact subspace of hausdorff space is closed*)

If str begins with a space, or contains digits:

str2="  Compact1Subspace2OfHausdorffSpace3IsClosed"
StringReplace[str2, RegularExpression["(?<=[a-z1-9])([A-Z])"]:> " $1" ]//ToLowerCase

(*  compact1 subspace2 of hausdorff space3 is closed*)
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