Thanks for clarifying—I think that I do take your meaning correctly now.

My issue comes with one subtle point, which is that, as you mention, we have a choice between incorporating a look-up table into our string, or using multiple substitutions; and the most naïve way of making use of the look-up table, as I did, turns the substitutions from s/fixed/fixed/ to something involving capturing parentheses. My question was thus: Can we get the best (or “apparently least computationally powerful”) of both worlds, by having just one substitution, but not using anything but the ordinary (theoretical-CS, not Perl-enhanced) regular-expression constructs—in particular, no back-references—in it?