#312 Function to compile a pattern

[Markus Elfring]

The class
library "http://java.sun.com/j2se/1.4.2/docs/api/java/ut
il/regex/Pattern.html" provides a method "compile".
Can a similar function be offered for TCL?
How do you think about to make the
function "Tcl_RegExpCompile"
(http://tcl.tk/man/tcl8.4/TclLib/RegExp.htm) callable
on the script level?

I would like to avoid the overhead for pattern
preprocessing for the
commands "http://tcl.tk/man/tcl8.4/TclCmd/regexp.htm
" and "http://tcl.tk/man/tcl8.4/TclCmd/regsub.htm" in
loops.
Can we get more performance if the additional memory
for the technique "Compile caching" will be saved?

[Jeffrey Hobbs]

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Tcl already does this if you keep the RE in an object (even a
static object as part of a compiled body of code). IOW, just
set the RE to a var, and use that var just as an RE.

[Markus Elfring]

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I assumed that "Compile caching" is performed already.
But how much processing does it need by TCL? This effort
might not be needed for constant patterns that should be
compiled only once before the matching is done in a loop.

[Donal K. Fellows]

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Fleshing out with an example:

set RE {the.*complex.*regexp}
# Match once against the empty string; this forces compilation
regexp $RE {}
# Now use $RE instead of the literal and you'll use the
# pre-compiled RE.

In fact, there's a few other places which do caching as
well. But IMHO the biggest advantage of the above technique
is that you can also give logical names to your RE (e.g.
$LinkTagMatcher instead {<a[^>]*?HREF="([^"]+)"}) and this
is a big win in maintenance and code-simplicity terms.

[Markus Elfring]

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I'm sorry - I did not know that TCL variables can be marked
that an instance contains a compiled regular expression.
Would you like to publish this behaviour on the
page "http://mini.net/tcl/regexp"
and "http://mini.net/tcl/2244"?

[Donal K. Fellows]

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You do it Marcus. You're obviously much keener on doing
this than I am especially as virtually everyone's already
reaping the benefits of precompilation anyway, one way or
another.

[Markus Elfring]

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I have updated these wikis.