expr: Regex not detecting valid expression

Question

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:

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?

Regular expressions only work within languages, shells use wildcards or globs, please google these terms for the shell you are using, **echo $SHELL ** — Arif Burhan
– Arif Burhan, Commented Mar 2, 2016 at 0:59
Thanks, didn't hear any information that expr uses globs. Though, I don't get what you mean by echo $SHELL, because I mentioned both in tags and in the hashbang that I'm using bash. — Nolan Akash
– Nolan Akash, Commented Mar 2, 2016 at 1:05
@ArifBurhan Also, I checked the man pages, and it says that the phrase expr STRING : REGEXP uses regular expressions. — Nolan Akash
– Nolan Akash, Commented Mar 2, 2016 at 1:14
echo $(...) is almost always redundant; there's no reason to capture the output of a command just to output it. — chepner
– chepner, Commented Mar 2, 2016 at 2:19
@chepner I commented that I wanted to see what It would produce. If I immediately put it into a script, it might have dangerous consequences. — Nolan Akash
– Nolan Akash, Commented Mar 2, 2016 at 2:22

chepner · Accepted Answer · 2016-03-02 03:16:56Z

1

A few things to note with expr:

The regular expression is implicitly anchored at the beginning of the string (as if you started the expression with ^).
The capture group is indicated with escaped parentheses, $...$.
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

edited Mar 2, 2016 at 3:16

answered Mar 2, 2016 at 2:23

chepner

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3 Comments

Nolan Akash Over a year ago

This gives the same error as before, so while some of this might help me, it still doesn't work. Did you test it out with the gist I had?

chepner Over a year ago

Sorry, tried to do too much from memory. Updating, but there is one caveat: you can only capture the first device that matches, not any more. (The non-greedy +? operator isn't supported, but would be unnecessary anyway.)

Nolan Akash Over a year ago

This works! I don't believe I will need to find other devices, but if I need to, I think I can use string manipulation to remove the captured line. Thanks so much!

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