'''`[http://www.tcl.tk/man/tcl/TclCmd/subst.htm%|%subst]`''', a [Tcl Commands%|%built-in] Tcl [command], [dodekalogue%|%substitutes] [variable substitution%|%variables] [command substitution%|%commands], and [dodekalogue%|%backslash sequences] in a [string]. ** Synopsis ** : '''subst''' ?'''-nobackslashes'''? ?'''-nocommands'''? ?'''-novariables'''? ''string'' ** Documentation ** [http://www.tcl.tk/man/tcl/TclCmd/subst.htm%|%official manpage%|%]: ** Description ** '''`subst`''' returns the given string with all [variable substitution%|%variables], [script substitution%|%commands], and [Dodekalogue%|%backslash sequences] substituted. It does not interpret the string as a [script], so quotes and braces don't have any effect. For example, in ====== subst {one two {[lindex {three four five} 1]}} ====== the result is ======none one two {four} ====== '''`-nobackslashes`''', '''`-nocommands`''', and '''`novariables`''', respectively inhibit the corresponding substitutions. `subst` and `[string map]` are handy for text-processing applications like [SGML] and [XML] transformations. Where in [Perl], "right-hand-side" variables with [regular expression] substitution are used, in Tcl `subst` is used to substitute in the result of a [regular expression%|%regular expression] match. ** Examples ** Simple example of using `subst` with [XML]/[HTML]. ====== set html {$title} set title {Hello, World!} set output [subst -nocommands $html] ;# -> Hello, World! ====== Another alternative would be to use [XPath]. ** Generating Scripts and Lists ** When using `subst` to generate a [word] in a [script] `[list]` should be used to [Tcl Quoting%|%quote] the word: ====== set value {Hello, World} set script [subst { puts [list $value] }] eval $script ====== The same is true when generating a list: ====== set item1 {some thing} set item3 {three different things} set list [subst { {a list item} $item3 [list $item1] {another list item} }] lindex $list 4 ;# -> {some thing} ====== ** Discussion ** [RS] most often uses `subst` for expanding [Unicode%|%Unicode characters]: cross-platform, in mostly 8-bit environments, it is most robust to output characters in the `\u....` notation. Such snippets can be pasted into a text widget and visualized by ====== subst [$t get 1.0 end] ====== ** Substitution and Evaluation ** Although `-nobackslashes`, `-nocommands`, and `-novariables`, inhibit substitutions by `subst` itself, the interpreter that does the evaluation necessary for a substitution is not affected by these options. This means, for example, that even when `-nocommands` is given, command occurs in the following example: ======none set var {code inclusion perverse $tcl_platform(os[puts OUCH!])} puts [subst -nocommands $var] ==> OUCH! ====== In the following example, even though `-novariable` is given, variable substitution is performed: ====== set var hello subst -novariables {[lindex $var]} ====== In the following example, even though `-nobackslashes` is given, backslash substitution is performed: ======none % subst -nobackslashes {$tcl_platform(threade\x64)\x64} 1\x64 ====== reference: [http://groups.google.fr/group/fr.comp.lang.tcl/msg/b0038cac3c0cfa04%|%Eric Hassold, fr.comp.lang.tcl, 2008-12-30%|%] reference: [https://core.tcl-lang.org/tcl/tktview/536838ffffffffffffff%|%Tcl bug 536838%|%] See also [double substitution]. ** History ** What changed in Tcl 8.4.0 with regards to how subst treats break and continue during command substitution? See [https://core.tcl-lang.org/tcl/tktview?name=536831%|%Tcl Bug 536831], [http://core.tcl.tk/tcl/tktview?name=684982%|%Tcl Feature Request 684982], and the changes in the tests subst-10.*. Without checking every byte, I think the incompatible changes are limited to those uses of `subst` that attempt command substitution on a string that is not a syntactically valid Tcl script -- arguably something no script should be doing anyway. ** Enhancement Suggestion: Hook for Variable Expansion ** [jcw] 2004-05-03: It would be useful to extend `subst` so it lets one catch variable accesses, and perhaps even command executions. What I mean is that when you `subst` text with "... $var ..." then sometimes it is useful to be able to intercept the expansion, by turning it into a call such as `myhandler var` for example, the result of which then gets used as substitution. The same (perhaps less important) might apply to `... [[cmd ...]] ...` expansions. This makes it simpler to implement tiny languages which also use `$var` and `$var(item)` as access mechanisms, but to things which are not necessarily stored in Tcl variables/arrays. Would it be an idea to extend subst so it optionally passes each of its substitutions to a command? Could be a "-command ..." option, or simply the presence of more args. '''[DGP]''' Am I missing something? Aren't you asking for variable and command [trace%|%traces], which exist? D'oh! I'm missing that in this case you want to set a trace on a whole set of variables/commands whose names you do not know. OK, something to think about... Anyhow, I think that's the right way to address the issue generally... add more types of traces that can be used everywhere. I'd be shy about diverging the implementation of `subst` from the implementation of the substitution portion of normal script evaluation. [jcw]: Yes, that's exactly the scenario. `subst` on a string to expand names which are not known up front. Looks like there is no way to catch this right now. Perhaps some new "unknown traces" (or whatever terminology) would indeed be better. The key is to intercept between the parse for var/cmd expansions and the lookup for existing ones. ** Parallel to double quoting ** [AMG]: Am I correct in my understanding that: ====== [subst {anything at all}] ====== is always equivalent to: ====== "anything at all" ====== for absolutely any value of "anything at all"? It occurred to me that if this is indeed the case, then maybe this equivalence could be the reason why backslash-newline-whitespace inside braces is replaced with a single space, in the interest of mirroring the way double quotes work. But then I experimented and found that `subst` internally will do this replacement and does not need the Tcl interpreter to preprocess its input in this way. [PYK]: Could you elaborate on what you mean by "internally will do this replacement"? [AMG]: `subst` internally replaces backslash-newline-whitespace with a single space. It's as simple as that. Here's a demonstration: ====== % subst abc\\\n\ \ def abc def ====== [PYK] 2020-11-10: The statement of equivalence is not true: ====== subst {"} ;# -> the double quote character lindex """ ;# -> error: extra characters after close-quote ====== ** See also ** [eval]: [regsub]: [string map]: [an extension to subst]: [Template and Macro processing]: ** Page Authors ** [Joe English]: [Lars H]: [pyk]: <> Arts and crafts of Tcl-Tk programming | Tcl syntax | Command | String Processing