http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/regsub.htm 
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[[Feel free to add various examples, demonstrating the use of the various
flags, etc.]]

One example of using regsub from [Brent Welch]'s [BOOK Practical Programming in Tcl and Tk] is:

 regsub -- {([^\.]*)\.c} file.c {cc -c & -o \1.o} ccCmd

The ''&'' is replaced by '''file.c''', and the ''\1'' is replaced by '''file'''.

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Recently on the Tcler's Wiki chat room, someone had the desire to convert
a string like this:

 rand ||=> this is some text <=|| rand
to
 rand ||=> some other text <=|| rand


 set unique1 {\|\|=>}
 set unique2 {<=\|\|}
 set string {rand ||=> this is some text <=|| rand}
 set replacement {some other text}
 set new [regsub -- "($unique1) .* ($unique2)" $string "\\1$replacement\\2" string ]
 puts $new
 puts $string

Note that the regular expression metacharacters in unique1 and unique2 need to be quoted so they are not treated as metacharacters.


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[AM] (7 october 2003) I asked about a complicated substitution in the chatroom:

Here is the question:

I have a fixed substring that delimits a variable number of characters. 
Anything in between (including the delimiters) must be replaced by a repetition
of another string. For example:

        1234A000aadA12234 --> 1234BXBXBXBX12234

(A000aadA is 8 characters, my replacing string fits 4 times in that)

arjen:   I do not think I can use some clever regexp to do this ... (note: things will always fit)

arjen:   The regexp to identify the substring could be: {A[^A]*A}

arjen:   But now to get the replacing string ...

CoderX2 easy... one sec

CoderX2 

   set string "1234A000aadA12234"
   set substring "BX"
   regsub -all {(A[^A]*A)} $string {[string repeat $substring [expr {[string length "\1"] / [string length $substring]}]]} new_string
   set new_string [subst $new_string]

(conversation edited to highlight this wonderful gem!)

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Has a '''-eval''' flag to regsub ever been suggested?  It would apply in the above example, and some other common idioms, e.g., url-deoding:

  regsub -all -eval {%([:xdigit:][:xdigit:]} $str {binary format H2 \1} str

The idea is that the replacement string gets eval-ed after expanding the \1 instead of just substituted in.  To safely do this otherwise needs an extra call to regsub before (to protect existing [[]]s) and a call to subst afterwards to do the evaluation.

-JR

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[elfring] 2003-10-29 TCL variables can be marked that an instance contains a compiled regular expression.
REs can be pre-compiled by the call "regexp $RE {}" [http://sourceforge.net/tracker/?group_id=10894&atid=360894&func=detail&aid=832230].

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