diff --git a/base/char.jl b/base/char.jl
index bc68a672ce0ca..2e8410f6903e2 100644
--- a/base/char.jl
+++ b/base/char.jl
@@ -223,6 +223,7 @@ hash(x::Char, h::UInt) =
     hash_uint64(((bitcast(UInt32, x) + UInt64(0xd4d64234)) << 32) ⊻ UInt64(h))
 
 first_utf8_byte(c::Char) = (bitcast(UInt32, c) >> 24) % UInt8
+first_utf8_byte(c::AbstractChar) = first_utf8_byte(Char(c)::Char)
 
 # fallbacks:
 isless(x::AbstractChar, y::AbstractChar) = isless(Char(x), Char(y))
diff --git a/base/strings/search.jl b/base/strings/search.jl
index b9c14f06e0898..9bd69ae2f8a03 100644
--- a/base/strings/search.jl
+++ b/base/strings/search.jl
@@ -10,7 +10,29 @@ match strings with [`match`](@ref).
 """
 abstract type AbstractPattern end
 
-nothing_sentinel(i) = i == 0 ? nothing : i
+# TODO: These unions represent bytes in memory that can be accessed via a pointer.
+# this property is used throughout Julia, e.g. also in IO code.
+# This deserves a better solution - see #53178.
+# If such a better solution comes in place, these unions should be replaced.
+const DenseInt8 = Union{
+    DenseArray{Int8},
+    FastContiguousSubArray{Int8,N,<:DenseArray} where N
+}
+
+# Note: This union is different from that above in that it includes CodeUnits.
+# Currently, this is redundant as CodeUnits <: DenseVector, but this subtyping
+# is buggy and may be removed in the future, see #54002
+const DenseUInt8 = Union{
+    DenseArray{UInt8},
+    FastContiguousSubArray{UInt8,N,<:DenseArray} where N,
+    CodeUnits{UInt8, <:Union{String, SubString{String}}},
+    FastContiguousSubArray{UInt8,N,<:CodeUnits{UInt8, <:Union{String, SubString{String}}}} where N,
+}
+
+const DenseUInt8OrInt8 = Union{DenseUInt8, DenseInt8}
+
+last_byteindex(x::Union{String, SubString{String}}) = ncodeunits(x)
+last_byteindex(x::DenseUInt8OrInt8) = lastindex(x)
 
 function last_utf8_byte(c::Char)
     u = reinterpret(UInt32, c)
@@ -30,11 +52,11 @@ function findnext(pred::Fix2{<:Union{typeof(isequal),typeof(==)},<:AbstractChar}
     end
     @inbounds isvalid(s, i) || string_index_err(s, i)
     c = pred.x
-    c ≤ '\x7f' && return nothing_sentinel(_search(s, c % UInt8, i))
+    c ≤ '\x7f' && return _search(s, first_utf8_byte(c), i)
     while true
         i = _search(s, first_utf8_byte(c), i)
-        i == 0 && return nothing
-        pred(s[i]) && return i
+        i === nothing && return nothing
+        isvalid(s, i) && pred(s[i]) && return i
         i = nextind(s, i)
     end
 end
@@ -47,31 +69,41 @@ const DenseBytes = Union{
     CodeUnits{UInt8, <:Union{String, SubString{String}}},
 }
 
-const ByteArray = Union{DenseBytes, DenseArrayType{Int8}}
+function findfirst(pred::Fix2{<:Union{typeof(isequal),typeof(==)},<:Union{UInt8, Int8}}, a::Union{DenseInt8, DenseUInt8})
+    findnext(pred, a, firstindex(a))
+end
 
-findfirst(pred::Fix2{<:Union{typeof(isequal),typeof(==)},<:Union{Int8,UInt8}}, a::ByteArray) =
-    nothing_sentinel(_search(a, pred.x))
+function findnext(pred::Fix2{<:Union{typeof(isequal),typeof(==)},UInt8}, a::DenseUInt8, i::Integer)
+    _search(a, pred.x, i)
+end
 
-findnext(pred::Fix2{<:Union{typeof(isequal),typeof(==)},<:Union{Int8,UInt8}}, a::ByteArray, i::Integer) =
-    nothing_sentinel(_search(a, pred.x, i))
+function findnext(pred::Fix2{<:Union{typeof(isequal),typeof(==)},Int8}, a::DenseInt8, i::Integer)
+    _search(a, pred.x, i)
+end
 
-findfirst(::typeof(iszero), a::ByteArray) = nothing_sentinel(_search(a, zero(UInt8)))
-findnext(::typeof(iszero), a::ByteArray, i::Integer) = nothing_sentinel(_search(a, zero(UInt8), i))
+# iszero is special, in that the bitpattern for zero for Int8 and UInt8 is the same,
+# so we can use memchr even if we search for an Int8 in an UInt8 array or vice versa
+findfirst(::typeof(iszero), a::DenseUInt8OrInt8) = _search(a, zero(UInt8))
+findnext(::typeof(iszero), a::DenseUInt8OrInt8, i::Integer) = _search(a, zero(UInt8), i)
 
-function _search(a::Union{String,SubString{String},<:ByteArray}, b::Union{Int8,UInt8}, i::Integer = 1)
-    if i < 1
+function _search(a::Union{String,SubString{String},DenseUInt8OrInt8}, b::Union{Int8,UInt8}, i::Integer = firstindex(a))
+    fst = firstindex(a)
+    lst = last_byteindex(a)
+    if i < fst
         throw(BoundsError(a, i))
     end
-    n = sizeof(a)
-    if i > n
-        return i == n+1 ? 0 : throw(BoundsError(a, i))
+    n_bytes = lst - i + 1
+    if i > lst
+        return i == lst+1 ? nothing : throw(BoundsError(a, i))
     end
-    p = pointer(a)
-    q = GC.@preserve a ccall(:memchr, Ptr{UInt8}, (Ptr{UInt8}, Int32, Csize_t), p+i-1, b, n-i+1)
-    return q == C_NULL ? 0 : Int(q-p+1)
+    GC.@preserve a begin
+        p = pointer(a)
+        q = ccall(:memchr, Ptr{UInt8}, (Ptr{UInt8}, Int32, Csize_t), p+i-fst, b, n_bytes)
+    end
+    return q == C_NULL ? nothing : (q-p+fst) % Int
 end
 
-function _search(a::ByteArray, b::AbstractChar, i::Integer = 1)
+function _search(a::DenseUInt8, b::AbstractChar, i::Integer = firstindex(a))
     if isascii(b)
         _search(a,UInt8(b),i)
     else
@@ -80,41 +112,51 @@ function _search(a::ByteArray, b::AbstractChar, i::Integer = 1)
 end
 
 function findprev(pred::Fix2{<:Union{typeof(isequal),typeof(==)},<:AbstractChar},
-                  s::String, i::Integer)
+                  s::Union{String, SubString{String}}, i::Integer)
     c = pred.x
-    c ≤ '\x7f' && return nothing_sentinel(_rsearch(s, c % UInt8, i))
+    c ≤ '\x7f' && return _rsearch(s, first_utf8_byte(c), i)
     b = first_utf8_byte(c)
     while true
         i = _rsearch(s, b, i)
-        i == 0 && return nothing
-        pred(s[i]) && return i
+        i == nothing && return nothing
+        isvalid(s, i) && pred(s[i]) && return i
         i = prevind(s, i)
     end
 end
 
-findlast(pred::Fix2{<:Union{typeof(isequal),typeof(==)},<:Union{Int8,UInt8}}, a::ByteArray) =
-    nothing_sentinel(_rsearch(a, pred.x))
+function findlast(pred::Fix2{<:Union{typeof(isequal),typeof(==)},<:Union{Int8,UInt8}}, a::DenseUInt8OrInt8)
+    findprev(pred, a, lastindex(a))
+end
 
-findprev(pred::Fix2{<:Union{typeof(isequal),typeof(==)},<:Union{Int8,UInt8}}, a::ByteArray, i::Integer) =
-    nothing_sentinel(_rsearch(a, pred.x, i))
+function findprev(pred::Fix2{<:Union{typeof(isequal),typeof(==)},Int8}, a::DenseInt8, i::Integer)
+    _rsearch(a, pred.x, i)
+end
 
-findlast(::typeof(iszero), a::ByteArray) = nothing_sentinel(_rsearch(a, zero(UInt8)))
-findprev(::typeof(iszero), a::ByteArray, i::Integer) = nothing_sentinel(_rsearch(a, zero(UInt8), i))
+function findprev(pred::Fix2{<:Union{typeof(isequal),typeof(==)},UInt8}, a::DenseUInt8, i::Integer)
+    _rsearch(a, pred.x, i)
+end
 
-function _rsearch(a::Union{String,ByteArray}, b::Union{Int8,UInt8}, i::Integer = sizeof(a))
-    if i < 1
-        return i == 0 ? 0 : throw(BoundsError(a, i))
+# See comments above for findfirst(::typeof(iszero)) methods
+findlast(::typeof(iszero), a::DenseUInt8OrInt8) = _rsearch(a, zero(UInt8))
+findprev(::typeof(iszero), a::DenseUInt8OrInt8, i::Integer) = _rsearch(a, zero(UInt8), i)
+
+function _rsearch(a::Union{String,SubString{String},DenseUInt8OrInt8}, b::Union{Int8,UInt8}, i::Integer = last_byteindex(a))
+    fst = firstindex(a)
+    lst = last_byteindex(a)
+    if i < fst
+        return i == fst - 1 ? nothing : throw(BoundsError(a, i))
+    end
+    if i > lst
+        return i == lst+1 ? nothing : throw(BoundsError(a, i))
     end
-    n = sizeof(a)
-    if i > n
-        return i == n+1 ? 0 : throw(BoundsError(a, i))
+    GC.@preserve a begin
+        p = pointer(a)
+        q = ccall(:memrchr, Ptr{UInt8}, (Ptr{UInt8}, Int32, Csize_t), p, b, i-fst+1)
     end
-    p = pointer(a)
-    q = GC.@preserve a ccall(:memrchr, Ptr{UInt8}, (Ptr{UInt8}, Int32, Csize_t), p, b, i)
-    return q == C_NULL ? 0 : Int(q-p+1)
+    return q == C_NULL ? nothing : (q-p+fst) % Int
 end
 
-function _rsearch(a::ByteArray, b::AbstractChar, i::Integer = length(a))
+function _rsearch(a::DenseUInt8, b::AbstractChar, i::Integer = length(a))
     if isascii(b)
         _rsearch(a,UInt8(b),i)
     else
@@ -224,18 +266,19 @@ end
 
 in(c::AbstractChar, s::AbstractString) = (findfirst(isequal(c),s)!==nothing)
 
-function _searchindex(s::Union{AbstractString,ByteArray},
+function _searchindex(s::Union{AbstractString,DenseUInt8OrInt8},
                       t::Union{AbstractString,AbstractChar,Int8,UInt8},
                       i::Integer)
+    sentinel = firstindex(s) - 1
     x = Iterators.peel(t)
     if isnothing(x)
-        return 1 <= i <= nextind(s,lastindex(s))::Int ? i :
+        return firstindex(s) <= i <= nextind(s,lastindex(s))::Int ? i :
                throw(BoundsError(s, i))
     end
     t1, trest = x
     while true
         i = findnext(isequal(t1),s,i)
-        if i === nothing return 0 end
+        if i === nothing return sentinel end
         ii = nextind(s, i)::Int
         a = Iterators.Stateful(trest)
         matched = all(splat(==), zip(SubString(s, ii), a))
@@ -509,9 +552,8 @@ julia> findall(UInt8[1,2], UInt8[1,2,3,1,2])
 !!! compat "Julia 1.3"
      This method requires at least Julia 1.3.
 """
-
-function findall(t::Union{AbstractString, AbstractPattern, AbstractVector{<:Union{Int8,UInt8}}},
-                 s::Union{AbstractString, AbstractPattern, AbstractVector{<:Union{Int8,UInt8}}},
+function findall(t::Union{AbstractString, AbstractPattern, AbstractVector{UInt8}},
+                 s::Union{AbstractString, AbstractPattern, AbstractVector{UInt8}},
                  ; overlap::Bool=false)
     found = UnitRange{Int}[]
     i, e = firstindex(s), lastindex(s)
@@ -564,7 +606,7 @@ function _rsearchindex(s::AbstractString,
     end
 end
 
-function _rsearchindex(s::String, t::String, i::Integer)
+function _rsearchindex(s::Union{String, SubString{String}}, t::Union{String, SubString{String}}, i::Integer)
     # Check for fast case of a single byte
     if lastindex(t) == 1
         return something(findprev(isequal(t[1]), s, i), 0)
diff --git a/test/strings/search.jl b/test/strings/search.jl
index 692286359868d..d8883bad24b48 100644
--- a/test/strings/search.jl
+++ b/test/strings/search.jl
@@ -155,6 +155,16 @@ for str in [u8str]
     @test findprev(isequal('ε'), str, 4) === nothing
 end
 
+# See the comments in #54579
+@testset "Search for invalid chars" begin
+    @test findfirst(==('\xff'), "abc\xffde") == 4
+    @test findprev(isequal('\xa6'), "abc\xa69", 5) == 4
+    @test isnothing(findfirst(==('\xff'), "abcdeæd"))
+
+    @test isnothing(findnext(==('\xa6'), "æ", 1))
+    @test isnothing(findprev(==('\xa6'), "æa", 2))
+end
+
 # string forward search with a single-char string
 @test findfirst("x", astr) === nothing
 @test findfirst("H", astr) == 1:1
@@ -445,6 +455,45 @@ end
             @test_throws BoundsError findprev(pattern, A, -3)
         end
     end
+
+    @test findall([0x01, 0x02], [0x03, 0x01, 0x02, 0x01, 0x02, 0x06]) == [2:3, 4:5]
+    @test isempty(findall([0x04, 0x05], [0x03, 0x04, 0x06]))
+end
+
+# Issue 54578
+@testset "No conflation of Int8 and UInt8" begin
+    # Work for mixed types if the values are the same
+    @test findfirst(==(Int8(1)), [0x01]) == 1
+    @test findnext(iszero, Int8[0, -2, 0, -3], 2) == 3
+    @test findfirst(Int8[1,4], UInt8[0, 2, 4, 1, 8, 1, 4, 2]) == 6:7
+    @test findprev(UInt8[5, 6], Int8[1, 9, 2, 5, 6, 3], 6) == 4:5
+
+    # Returns nothing for the same methods if the values are different,
+    # even if the bitpatterns are the same
+    @test isnothing(findfirst(==(Int8(-1)), [0xff]))
+    @test isnothing(findnext(isequal(0xff), Int8[-1, -2, -1], 2))
+    @test isnothing(findfirst(UInt8[0xff, 0xfe], Int8[0, -1, -2, 1, 8, 1, 4, 2]))
+    @test isnothing(findprev(UInt8[0xff, 0xfe], Int8[1, 9, 2, -1, -2, 3], 6))
+end
+
+@testset "DenseArray with offsets" begin
+    isdefined(Main, :OffsetDenseArrays) || @eval Main include("../testhelpers/OffsetDenseArrays.jl")
+    OffsetDenseArrays = Main.OffsetDenseArrays
+
+    A = OffsetDenseArrays.OffsetDenseArray(collect(0x61:0x69), 100)
+    @test findfirst(==(0x61), A) == 101
+    @test findlast(==(0x61), A) == 101
+    @test findfirst(==(0x00), A) === nothing
+
+    @test findfirst([0x62, 0x63, 0x64], A) == 102:104
+    @test findlast([0x63, 0x64], A) == 103:104
+    @test findall([0x62, 0x63], A) == [102:103]
+
+    @test findfirst(iszero, A) === nothing
+    A = OffsetDenseArrays.OffsetDenseArray([0x01, 0x02, 0x00, 0x03], -100)
+    @test findfirst(iszero, A) == -97
+    @test findnext(==(0x02), A, -99) == -98
+    @test findnext(==(0x02), A, -97) === nothing
 end
 
 # issue 32568
diff --git a/test/testhelpers/OffsetDenseArrays.jl b/test/testhelpers/OffsetDenseArrays.jl
new file mode 100644
index 0000000000000..44a1b8d627800
--- /dev/null
+++ b/test/testhelpers/OffsetDenseArrays.jl
@@ -0,0 +1,31 @@
+"""
+    module OffsetDenseArrays
+
+A minimal implementation of an offset array which is also <: DenseArray.
+"""
+module OffsetDenseArrays
+
+struct OffsetDenseArray{A <: DenseVector, T} <: DenseVector{T}
+    x::A
+    offset::Int
+end
+OffsetDenseArray(x::AbstractVector{T}, i::Integer) where {T} = OffsetDenseArray{typeof(x), T}(x, Int(i))
+
+Base.size(x::OffsetDenseArray) = size(x.x)
+Base.pointer(x::OffsetDenseArray) = pointer(x.x)
+
+function Base.getindex(x::OffsetDenseArray, i::Integer)
+    @boundscheck checkbounds(x.x, i - x.offset)
+    x.x[i - x.offset]
+end
+
+function Base.setindex(x::OffsetDenseArray, v, i::Integer)
+    @boundscheck checkbounds(x.x, i - x.offset)
+    x.x[i - x.offset] = v
+end
+
+IndexStyle(::Type{<:OffsetDenseArray}) = Base.IndexLinear()
+Base.axes(x::OffsetDenseArray) = (x.offset + 1 : x.offset + length(x.x),)
+Base.keys(x::OffsetDenseArray) = only(axes(x))
+
+end # module