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regextclregexp-replaceregexp-substr

Having issue with back reference in TCL

I have the following code:

set a "10.20.30.40"
regsub -all {.([0-9]+).([0-9]+).} $a {\2 \1} b

I am trying to grep 2nd and 3rd octet of the IP address.

Expected output:

20 30

Actual output:

20 04 0

What is my mistake here?

Solution

  • You need to set the variables for the match and captured groups, then you can access them. Here is an example:

    set a "10.20.30.40"
set rest [regexp {[0-9]+\.([0-9]+)\.([0-9]+)\.[0-9]+} $a match submatch1 submatch2]
puts $submatch1
puts $submatch2

    Output of the demo

    20
30

    EDIT:

    You can use regsub and backerferences this way (I am now replacing the 3rd and 2nd octets, just for demonstration). Note that a literal dot must be escaped:

    set a "10.20.30.40"
regsub -all {\.([0-9]+)\.([0-9]+)\.} $a {.\2.\1.} b
puts $b

    Output of the demo: 

    10.30.20.40

    To obtain a "20 30" string, you need to use 

    regsub -all {^[0-9]+\.([0-9]+)\.([0-9]+)\.[0-9]+$} $a {\1 \2} b