问题
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?
回答1:
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
回答2:
I'd stay away from regular expressions altogether:
set b [join [lrange [split $a .] 1 2]]
Split the value on dots, take the 2nd and 3nd elements, and join them with a space.
来源：https://stackoverflow.com/questions/30320793/having-issue-with-back-reference-in-tcl