extract text between the two blocks using regex

With some obvious assumptions, and keeping it close to the pattern in the question:

perl -0777 -wnE'
    @pods = /Non-terminated\s+Pods:\s+\([0-9]+\s+in\s+total\)\n(.*?)\nAllocated resources:/gs;
    say for @pods
' input-file

(note modifiers on the regex in this line, which is too wide to fit on screen: /gs)

The regex from the question works when used instead of the one in this answer (and with no /s modifier, as it should) on a single block of text. To work with multiple blocks the (.*) in it need be changed to (.*?), so that it doesn't match all the way to the last Allocated...

The question doesn't say how precisely is that regex "used with perl"; I can't say what failed.

Comments on the command-line program above:

• The -0777 switch makes it read the file whole into a string, available in the program in the variable $_, to which the regex is bound by default

  There is also the switch -g, an alias for -0777, available starting with 5.36.0

• We still need the -n switch so that the program iterates over the "lines" of input (STDIN or a file). In this case the input record separator is undefined so it's all just one "line"

• The regex captures are returned since the match operator is in the list context, being assigned to the array @pods

Using gnu-grep you can use your regex with some tweaks:

kubectl describe  node |
grep -zoP '(?s)Non-terminated Pods:.*?in total.\R\K(.*?)(?=Allocated resources)'

  Namespace                   Name                                        CPU Requests  CPU Limits  Memory Requests  Memory Limits  Age
  ---------                   ----                                        ------------  ----------  ---------------  -------------  ---
  default                     foo                                         0 (0%)        0 (0%)      0 (0%)           0 (0%)         105s
  kube-system                 nginx-proxy-kube-worker-1                   25m (1%)      0 (0%)      32M (1%)         0 (0%)         9m8s
  kube-system                 nodelocaldns-xbjp8                          100m (5%)     0 (0%)      70Mi (4%)        170Mi (10%)    7m4s

• Used \K (match reset) after \R to remove that line from output
• Used -z option to treat treat input and output data as sequences of lines, each terminated by a zero byte (the ASCII NUL character) instead of a newline.

PS: Same regex will work with second input block as well with header line shown before each block.

Alternatively you can use any version sed for this job as well:

kubectl describe  node |
sed -n '/Non-terminated Pods:.*in total.*/,/Allocated resources:/ {//!p;}'

  Namespace                   Name                                        CPU Requests  CPU Limits  Memory Requests  Memory Limits  Age
  ---------                   ----                                        ------------  ----------  ---------------  -------------  ---
  default                     foo                                         0 (0%)        0 (0%)      0 (0%)           0 (0%)         105s
  kube-system                 nginx-proxy-kube-worker-1                   25m (1%)      0 (0%)      32M (1%)         0 (0%)         9m8s
  kube-system                 nodelocaldns-xbjp8                          100m (5%)     0 (0%)      70Mi (4%)        170Mi (10%)    7m4s

With your shown samples, please try following GNU awk code. Written and tested in GNU awk. Simple explanation would be, setting RS as Non-terminated Pods:.*Allocated resources: for Input_file. Then in main program checking if RT is NOT NULL then using gsub function of awk to substitute (^|\n)Non-terminated Pods:[^\n]*\n OR \nAllocated resources:\n* with NULL in RT variable and then printing its value which will provide output as per shown samples.

awk -v RS='Non-terminated Pods:.*Allocated resources:' '
RT{
  gsub(/(^|\n)Non-terminated Pods:[^\n]*\n|\nAllocated resources:\n*/,"",RT)
  print RT
}
'  Input_file

A possible solution could be as following for a very big files to read line by line.

Select range of lines of interest and remove the last one which is not included into desired output.

use strict;
use warnings;

while(<>) {
    if( /^  Namespace/ .. /^Allocated resources:/ ) {
        print unless /^Allocated resources:/;
    }
}

exit 0;

Output

  Namespace                   Name                                        CPU Requests  CPU Limits  Memory Requests  Memory Limits  Age
  ---------                   ----                                        ------------  ----------  ---------------  -------------  ---
  default                     foo                                         0 (0%)        0 (0%)      0 (0%)           0 (0%)         105s
  kube-system                 nginx-proxy-kube-worker-1                   25m (1%)      0 (0%)      32M (1%)         0 (0%)         9m8s
  kube-system                 nodelocaldns-xbjp8                          100m (5%)     0 (0%)      70Mi (4%)        170Mi (10%)    7m4s
  Namespace                   Name                                        CPU Requests  CPU Limits  Memory Requests  Memory Limits  Age
  ---------                   ----                                        ------------  ----------  ---------------  -------------  ---
  default                     foo-1                                       0 (0%)        0 (0%)      0 (0%)           0 (0%)         105s
  kube-system                 nginx-proxy-kube-worker-2                   25m (1%)      0 (0%)      32M (1%)         0 (0%)         9m8s
  kube-system                 nodelocaldns-xbjp3-2                        100m (5%)     0 (0%)      70Mi (4%)        170Mi (10%)    7m4s