Add findfirst, findlast for searching char in strings

NEWS.md

@@ -20,6 +20,8 @@ Build system changes
 New library functions
 ---------------------
 
+* `findfirst`, `findlast`, `findnext` and `findprev` now accept a character as first argument
+  to search for that character in a string passed as the second argument ([#31664]).
 * New `findall(pattern, string)` method where `pattern` is a string or regex ([#31834]).
 
 Standard library changes

base/strings/search.jl

@@ -104,6 +104,25 @@ julia> findfirst("Julia", "JuliaLang")
 findfirst(pattern::AbstractString, string::AbstractString) =
     findnext(pattern, string, firstindex(string))
 
+"""
+    findfirst(ch::AbstractChar, string::AbstractString)
+
+Find the first occurrence of character `ch` in `string`.
+
+!!! compat "Julia 1.3"
+    This function requires at least Julia 1.3.
+
+# Examples
+```jldoctest
+julia> findfirst('a', "happy")
+2
+
+julia> findfirst('z', "happy") === nothing
+true
+```
+"""
+findfirst(ch::AbstractChar, string::AbstractString) = findfirst(==(ch), string)
+
 # AbstractString implementation of the generic findnext interface
 function findnext(testf::Function, s::AbstractString, i::Integer)
     z = ncodeunits(s) + 1
@@ -255,6 +274,26 @@ julia> findnext("Lang", "JuliaLang", 2)
 """
 findnext(t::AbstractString, s::AbstractString, i::Integer) = _search(s, t, i)
 
+"""
+    findnext(ch::AbstractChar, string::AbstractString, start::Integer)
+
+Find the next occurrence of character `ch` in `string` starting at position `start`.
+
+!!! compat "Julia 1.3"
+    This function requires at least Julia 1.3.
+
+# Examples
+```jldoctest
+julia> findnext(`z`, "Hello to the world", 1) === nothing
+true
+
+julia> findnext(`o`, "Hello to the world", 6)
+8
+```
+"""
+findnext(ch::AbstractChar, string::AbstractString, ind::Integer) =
+    findnext(==(ch), string, ind)
+
 """
     findlast(pattern::AbstractString, string::AbstractString)
 
@@ -273,6 +312,25 @@ julia> findfirst("Julia", "JuliaLang")
 findlast(pattern::AbstractString, string::AbstractString) =
     findprev(pattern, string, lastindex(string))
 
+"""
+    findlast(ch::AbstractChar, string::AbstractString)
+
+Find the last occurrence of character `ch` in `string`.
+
+!!! compat "Julia 1.3"
+    This function requires at least Julia 1.3.
+
+# Examples
+```jldoctest
+julia> findlast('p', "happy")
+4
+
+julia> findlast('z', "happy") === nothing
+true
+```
+"""
+findlast(ch::AbstractChar, string::AbstractString) = findlast(==(ch), string)
+
 # AbstractString implementation of the generic findprev interface
 function findprev(testf::Function, s::AbstractString, i::Integer)
     if i < 1
@@ -428,6 +486,26 @@ julia> findprev("Julia", "JuliaLang", 6)
 """
 findprev(t::AbstractString, s::AbstractString, i::Integer) = _rsearch(s, t, i)
 
+"""
+    findprev(ch::AbstractChar, string::AbstractString, start::Integer)
+
+Find the previous occurrence of character `ch` in `string` starting at position `start`.
+
+!!! compat "Julia 1.3"
+    This function requires at least Julia 1.3.
+
+# Examples
+```jldoctest
+julia> findprev('z', "Hello to the world", 18) === nothing
+true
+
+julia> findprev('o', "Hello to the world", 18)
+15
+```
+"""
+findprev(ch::AbstractChar, string::AbstractString, ind::Integer) =
+    findprev(==(ch), string, ind)
+
 """
     occursin(needle::Union{AbstractString,Regex,AbstractChar}, haystack::AbstractString)
 

test/strings/search.jl

@@ -43,6 +43,28 @@ for str in [astr, GenericString(astr)]
     @test_throws BoundsError findnext(isequal('a'), str, nextind(str,lastindex(str))+1)
 end
 
+for str in [astr, GenericString(astr)]
+    @test_throws BoundsError findnext('z', str, 0)
+    @test_throws BoundsError findnext('∀', str, 0)
+    @test findfirst('x', str) == nothing
+    @test findfirst('\0', str) == nothing
+    @test findfirst('\u80', str) == nothing
+    @test findfirst('∀', str) == nothing
+    @test findfirst('H', str) == 1
+    @test findfirst('l', str) == 3
+    @test findfirst('e', str) == 2
+    @test findfirst('u', str) == nothing
+    @test findnext('l', str, 4) == 4
+    @test findnext('l', str, 5) == 11
+    @test findnext('l', str, 12) == nothing
+    @test findfirst(',', str) == 6
+    @test findnext(',', str, 7) == nothing
+    @test findfirst('\n', str) == 14
+    @test findnext('\n', str, 15) == nothing
+    @test_throws BoundsError findnext('ε', str, nextind(str,lastindex(str))+1)
+    @test_throws BoundsError findnext('a', str, nextind(str,lastindex(str))+1)
+end
+
 # ascii backward search
 for str in [astr]
     @test findlast(isequal('x'), str) == nothing
@@ -61,6 +83,23 @@ for str in [astr]
     @test findlast(isequal('\n'), str) == 14
 end
 
+for str in [astr]
+    @test findlast('x', str) == nothing
+    @test findlast('\0', str) == nothing
+    @test findlast('\u80', str) == nothing
+    @test findlast('∀', str) == nothing
+    @test findlast('H', str) == 1
+    @test findprev('H', str, 0) == nothing
+    @test findlast('l', str) == 11
+    @test findprev('l', str, 5) == 4
+    @test findprev('l', str, 4) == 4
+    @test findprev('l', str, 3) == 3
+    @test findprev('l', str, 2) == nothing
+    @test findlast(',', str) == 6
+    @test findprev(',', str, 5) == nothing
+    @test findlast('\n', str) == 14
+end
+
 # utf-8 forward search
 for str in (u8str, GenericString(u8str))
     @test_throws BoundsError findnext(isequal('z'), str, 0)