CODEWITHSUNDEEP
regexbashexpr
Nolan Akash
Nolan Akash

Reputation: 1094

expr: Regex not detecting valid expression

I'm trying to create a script that automatically looks for plugged in devices and makes a compressed backup of it. However, I'm having trouble finding the correct way on how to use expr:

#!/bin/bash

MountText=`mount`
# Show result of regex search
expr "$MountText" : '\/dev\/(sd[^a])\d on [\S]+\/[\s\S]+? type'

The expression by itself is \/dev\/(sd[^a])\d on [\S]+\/[\s\S]+? type, and captures the device name (sd*), while excluding mounts relating to sda.

I drafted the regex on Regexr (regex shared in link), and used what mount dumped (gist).

For some reason, it only gives this odd error:

0

I looked around, and I found this SO question. It didn't help me too much, because now it's implying that expr didn't recognize the parentheses I used to capture the device, and it also believed that the expression didn't capture anything!

I'm really confused. What am I doing wrong?

Upvotes: 0

Views: 1218

Answers (1)

chepner
chepner

Reputation: 532053

A few things to note with expr:

  1. The regular expression is implicitly anchored at the beginning of the string (as if you started the expression with ^).
  2. The capture group is indicated with escaped parentheses, \(...\).
  3. Only basic regular expressions are supported. Specifically, \s, \S, and +? are not supported.

The following will match the one device.

expr "$MountText" : '.*/dev/\(sd[^a]\)[[:digit:]] on '

Note that you don't need to use expr with bash, which has regular-expression matching built in.

regex='/dev/(sd[^a])[[:digit:]] on '
mount | while IFS= read -r line; do
    [[ $line =~ $regex ]] && echo ${BASH_REMATCH[1]}
done

Upvotes: 1

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