Closed
Description
While trying to create documentation for SubstitutionString (#26497), I found that replace using SubstitutionStrings doesn't work if the substition string has a newline or other escape sequence in it.
julia> msg = "#Hello# from Julia";
julia> re = r"#(.+)# from (?<name>\w+)";
julia> subst = s"FROM: \g<name>\n MESSAGE: \1"
s"FROM: \\g<name>\\n MESSAGE: \\1"
julia> replace(msg, re => subst)
ERROR: Bad replacement string: FROM: \g<name>\n MESSAGE: \1
Stacktrace:
[1] error at .\error.jl:33 [inlined]
[2] replace_err at .\regex.jl:241 [inlined]
[3] _replace(::Base.GenericIOBuffer{Array{UInt8,1}}, ::Base.SubstitutionString{String}, ::String, ::UnitRange{Int64}, ::Regex) at .\regex.jl:302
[4] #replace#350(::Int64, ::Function, ::String, ::Pair{Regex,Base.SubstitutionString{String}}) at .\strings\util.jl:427
[5] replace(::String, ::Pair{Regex,Base.SubstitutionString{String}}) at .\strings\util.jl:415
[6] top-level scope
julia> subst = s"FROM: \g<name>; MESSAGE: \1"
s"FROM: \\g<name>; MESSAGE: \\1"
julia> replace(msg, re => subst) #works fine when \n is removed
"FROM: Julia; MESSAGE: Hello"
Looking at the code, when the _replace function (in regex.jl) sees a backslash, it expects another backslash, a digit or a 'g', and throws an error with replace_err if it doesn't see one of those. Another consequence of this is that the replacement string can't contain things like Unicode codepoint sequences (\u0B85).
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