use Char replacement function for Charpattern

NEWS.md

@@ -203,6 +203,11 @@ Breaking changes
 
 This section lists changes that do not have deprecation warnings.
 
+  * `replace(s::AbstractString, pat=>repl)` for function `repl` arguments formerly
+    passed a substring to `repl` in all cases.  It now passes substrings for
+    string patterns `pat`, but a `Char` for character patterns (when `pat` is a
+    `Char`, collection of `Char`, or a character predicate) ([#25815]).
+
   * `readuntil` now does *not* include the delimiter in its result, matching the
     behavior of `readline`. Pass `keep=true` to get the old behavior ([#25633]).
 

base/strings/util.jl

@@ -365,6 +365,8 @@ end
 _replace(io, repl, str, r, pattern) = print(io, repl)
 _replace(io, repl::Function, str, r, pattern) =
     print(io, repl(SubString(str, first(r), last(r))))
+_replace(io, repl::Function, str, r, pattern::Function) =
+    print(io, repl(str[first(r)]))
 
 replace(str::String, pat_repl::Pair{Char}; count::Integer=typemax(Int)) =
     replace(str, equalto(first(pat_repl)) => last(pat_repl); count=count)
@@ -413,8 +415,10 @@ end
 Search for the given pattern `pat` in `s`, and replace each occurrence with `r`.
 If `count` is provided, replace at most `count` occurrences.
 `pat` may be a single character, a vector or a set of characters, a string,
-or a regular expression. If `r`
-is a function, each occurrence is replaced with `r(s)` where `s` is the matched substring.
+or a regular expression.
+If `r` is a function, each occurrence is replaced with `r(s)`
+where `s` is the matched substring (when `pat`is a `Regex` or `AbstractString`) or
+character (when `pat` is a `Char` or a collection of `Char`).
 If `pat` is a regular expression and `r` is a `SubstitutionString`, then capture group
 references in `r` are replaced with the corresponding matched text.
 To remove instances of `pat` from `string`, set `r` to the empty `String` (`""`).

test/strings/util.jl

@@ -261,6 +261,14 @@ end
 
     # Issue 25741
     @test replace("abc", ['a', 'd'] => 'A') == "Abc"
+
+    # for Char pattern call Char replacement function
+    @test replace("a", "a" => typeof) == "SubString{String}"
+    @test replace("a", r"a" => typeof) == "SubString{String}"
+    @test replace("a", 'a' => typeof) == "Char"
+    @test replace("a", occursin("a") => typeof) == "Char"
+    @test replace("a", ['a'] => typeof) == "Char"
+
 end
 
 @testset "chomp/chop" begin