Question

Is it possible to have do different substitutions in an expression using regsub?

example:

set a ".a/b.c..d/e/f//g"

Now, in this expression, is it possible to substitute
“.” as “yes”
“..” as “no”
“/” as “true”
“//” as “false” in a single regsub command?

Answer 1

With a regsub, no. There’s a long-standing feature request for this sort of thing (which requires substitution with the result of evaluating a command on the match information) but it’s not been acted on to date.

But you can use string map to do what you want in this case:

set a ".a/b.c..d/e/f//g"
set b [string map {".." "no" "." "yes" "//" "false" "/" "true"} $a]
puts "changed $a to $b"
# changed .a/b.c..d/e/f//g to yesatruebyescnodtrueetrueffalseg

Note that when building the map, if any from-value is a prefix of another, the longer from-value should be put first. (This is because the string map implementation checks which change to make in the order you list them in…)

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It’s possible to use regsub and subst to do multiple-target replacements in a two-step process, but I don’t advise it for anything other than very complex cases! A nice string map is far easier to work with.