Bash - numbers of multiple lines matching regex (possible oneliner?) - regex

Question

I'm not very fluent in bash but actively trying to improve, so I'd like to ask some experts here for a little suggestion :)
Let's say I've got a following text file:
Some
spam
about which I don't care.
I want following letters:
X1
X2
X3
I do not want these:
X4
X5
Nor this:
X6
But I'd like these, too:
I want following letters:
X7
And so on...
And I'd like to get numbers of lines with these letters, so my desired output should look like:
5 6 7 15
To clarify: I want all lines matching some regex /\s*X./, that occur right after one match with another regex /\sI want following letters:/
Right now I've got a working solution, which I don't really like:
cat data.txt | grep -oPz "\sI want following letters:((\s*X.)*)" | grep -oPz "\s*X." > tmp.txt
for entry in $(cat tmp.txt); do
grep -n $entry data.txt | cut -d ":" -f1
done
My question is: Is there any smart way, any tool I don't know with a functionality to do this in one line? (I esspecially don't like having to use temp file and a loop here)

Answer 1

You can use awk:
awk '/I want following/{p=1;next}!/^X/{p=0;next}p{print NR}' file
Explanation in multiline version:
#!/usr/bin/awk
/I want following/{
# Just set a flag and move on with the next line
p=1
next
}
!/^X/ {
# On all other lines that doesn't start with a X
# reset the flag and continue to process the next line
p=0
next
}
p {
# If the flag p is set it must be a line with X+number.
# print the line number NR
print NR
}

Answer 2

Following may help you here.
awk '!/X[0-9]+/{flag=""} /I want following letters:/{flag=1} flag' Input_file
Above will print the lines which have I want following letters: too in case you don't want these then use following.
awk '!/X[0-9]+/{flag=""} /I want following letters:/{flag=1;next} flag' Input_file
To add line number to output use following.
awk '!/X[0-9]+/{flag=""} /I want following letters:/{flag=1;next} flag{print FNR}' Input_file

Answer 3

First, let's optimize a little bit your current script:
#!/bin/bash
FILE="data.txt"
while read -r entry; do
[[ $entry ]] && grep -n $entry "$FILE" | cut -d ":" -f1
done < <(grep -oPz "\sI want following letters:((\s*X.)*)" "$FILE"| grep -oPz "\s*X.")
And here's some comments:
No need to use cat file|grep ... => grep ... file
Do not use the syntaxe for i in $(command), it's often the cause of multiple bugs and there's always a smarter solution.
No need to use a tmp file either
And then, there's a lot of shorter possible solutions. Here's one using awk:
$ awk '{ if($0 ~ "I want following letters:") {s=1} else if(!($0 ~ "^X[0-9]*$")) {s=0}; if (s && $0 ~ "^X[0-9]*$") {gsub("X", ""); print}}' data.txt
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