What version of Julia are you using?
julia> versioninfo()
Julia Version 1.7.0-rc1
Commit 9eade6195e (2021-09-12 06:45 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: Intel(R) Core(TM) i7-3770K CPU @ 3.50GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-12.0.1 (ORCJIT, ivybridge)
Environment:
JULIA_REVISE_INCLUDE = 1
julia> replace("aabbccdd","a"=>"y","c"=>"z")
"yybbzzdd"