titlecase: chars not starting a word can be converted to lowercase

* A second argument `strict` is added to `titlecase` to control
  whether to convert those chars to lowercase.
  This is useful e.g. for #23379.
* The one-arg version is deprecated, and will be equivalent to
  the new behavior (`strict=true`) in the future.
  This is to be compatible with the `istitle` function, so that
  `istitle(titlecase(s)) == true` when `s` has at least 1 letter.
  This is also how some languages (e.g. python) implement it, and
  is compatible with http://www.unicode.org/L2/L1999/99190.htm.

NEWS.md

@@ -329,6 +329,14 @@ Deprecated or removed
   * `diagm(A::SparseMatrixCSC)` has been deprecated in favor of
     `spdiagm(sparsevec(A))` ([#23341]).
 
+  * `titlecase(s)` is deprecated in favor of `titlecase(s, #=strict=# false)`,
+    which means to preserve uppercase characters; in the future, the second
+    argument will default to true, i.e. every character not starting a word
+    will be converted to lowercase.
+    This is to be consistent with the function `istitle` (so that
+    `istitle(titlecase(s))` is always true if `s` contains at least one letter),
+    and with other languages.
+
 Command-line option changes
 ---------------------------
 

base/deprecated.jl

@@ -1708,6 +1708,9 @@ export hex2num
 # PR 23341
 @deprecate diagm(A::SparseMatrixCSC) spdiagm(sparsevec(A))
 
+# PR #23393
+@deprecate titlecase(s::AbstractString) titlecase(s, false)
+
 # END 0.7 deprecations
 
 # BEGIN 1.0 deprecations

base/strings/basic.jl

@@ -406,27 +406,32 @@ julia> lowercase("STRINGS AND THINGS")
 lowercase(s::AbstractString) = map(lowercase, s)
 
 """
-    titlecase(s::AbstractString)
+    titlecase(s::AbstractString, strict::Bool)
 
-Capitalizes the first character of each word in `s`.
+Capitalizes the first character of each word in `s`;
+if `strict` is true, every other character is
+converted to lowercase, otherwise they are left unchanged.
 See also [`ucfirst`](@ref) to capitalize only the first
 character in `s`.
 
 # Examples
 ```jldoctest
-julia> titlecase("the julia programming language")
+julia> titlecase("the JULIA programming language", true)
 "The Julia Programming Language"
+
+julia> titlecase("ISS - international space station", false)
+"ISS - International Space Station"
 ```
 """
-function titlecase(s::AbstractString)
+function titlecase(s::AbstractString, strict::Bool)
     startword = true
     b = IOBuffer()
     for c in s
         if isspace(c)
             print(b, c)
             startword = true
         else
-            print(b, startword ? titlecase(c) : c)
+            print(b, startword ? titlecase(c) : strict ? lowercase(c) : c)
             startword = false
         end
     end

test/strings/basic.jl

@@ -417,10 +417,12 @@ end
 @test ucfirst(GenericString("A")) == "A"
 
 # titlecase
-@test titlecase('lj') == 'Lj'
-@test titlecase("ljubljana") == "Ljubljana"
-@test titlecase("aBc ABC") == "ABc ABC"
-@test titlecase("abcD   EFG\n\thij") == "AbcD   EFG\n\tHij"
+@test titlecase('lj', true) == 'Lj'
+@test titlecase("ljubljana", true) == "Ljubljana"
+@test titlecase("aBc ABC", true)  == "ABc ABC"
+@test titlecase("aBc ABC", false) == "Abc Abc"
+@test titlecase("abcD   EFG\n\thij", true)  == "AbcD   EFG\n\tHij"
+@test titlecase("abcD   EFG\n\thij", false) == "Abcd   Efg\n\tHij"
 
 # issue # 11464: uppercase/lowercase of GenericString becomes a String
 str = "abcdef\uff\uffff\u10ffffABCDEF"