diff --git a/base/strings/substring.jl b/base/strings/substring.jl
index 2c701a03d0633..86efbc1f6df3e 100644
--- a/base/strings/substring.jl
+++ b/base/strings/substring.jl
@@ -55,6 +55,11 @@ convert(::Type{SubString{S}}, s::AbstractString) where {S<:AbstractString} =
     SubString(convert(S, s))
 convert(::Type{T}, s::T) where {T<:SubString} = s
 
+# Regex match allows only Union{String, SubString{String}} so define conversion to this type
+convert(::Type{Union{String, SubString{String}}}, s::String) = s
+convert(::Type{Union{String, SubString{String}}}, s::SubString{String}) = s
+convert(::Type{Union{String, SubString{String}}}, s::AbstractString) = convert(String, s)
+
 function String(s::SubString{String})
     parent = s.string
     copy = GC.@preserve parent unsafe_string(pointer(parent, s.offset+1), s.ncodeunits)