Issue
Using the (?<=_)(.*)(?=\.) regex with the 23353_test.txt test string returns nothing with grep with the -p option. It doesn’t show errors either. I expect the return to be test. But when the regex is tried in regex101.com it runs correctly.
Solution
The following GNU grep command extracts the right substring:
grep -oP '(?<=_).*(?=\.)' file
Note that .* matches greedily, and if you want to make sure you match a substring between the closest _ and . you need to use a
grep -oP '(?<=_)[^._]*(?=\.)' file
where [^._]* matches zero or more chars other than . and _.
If you cannot rely on your grep, you can use sed here:
sed -n 's/.*_\(.*\)\..*/\1/p' file
See the online demo:
#!/bin/bash
s='23353_test.txt'
grep -oP '(?<=_)(.*)(?=\.)' <<< "$s"
# => test
sed -n 's/.*_\(.*\)\..*/\1/p' <<< "$s"
# => test
Answered By - Wiktor Stribiżew Answer Checked By - Cary Denson (WPSolving Admin)
