'Chi_2'   =~ /\bChi_2\b/  # Match
'!Chi_2!' =~ /\bChi_2\b/  # Match

##</code><code>##

($_ = '\\Chi+3'  ) =~ s/\\Ch\b/$ch/g;      # Won't replace
($_ = '\\Chi+3'  ) =~ s/\\Chi\b/$chi/g;    # Will replace
($_ = '\\Chi_2+3') =~ s/\\Chi\b/$chi/g;    # Won't replace

##</code><code>##

($_ = '\\Chi+3'  ) =~ s/\\Ch???/$ch/g;    # Won't replace
($_ = '\\Chi+3'  ) =~ s/\\Chi???/$chi/g;  # Will replace
($_ = '\\Chi_2+3') =~ s/\\Chi???/$chi/g;  # Will replace

##</code><code>##

/\\([a-zA-Z]+)/ exists($vars{$1}) ? $vars{$1} : "\\$1" /eg