Why is my regex failing in shell script although it is working in jregex?

Question

I have written this simple script for matching in shell script

file_name="xyz_abc_diagnostics.wifi2.2015-07-30.12-30-52.tar.gz"
chk_regex=".*\.\d+\-\d+\-\d+\.\d+\-\d+\-\d+.*"
if [[ "$file_name" =~ $chk_regex  ]];then
echo "in obs regex"
else
echo "dont triggered"
fi

I have checked this regex in java and here it is working fine. my syntax is right because when i use

.*

it is working fine. For shell script regex testing i have use this site http://regexraptor.net/ to check it also don't matching but in https://regex101.com/ which uses java regex it matches. I am not able to understand why it is failing in shell script. Is there any difference in shell script regex?If yes then please suggest me what changes i have to make

Tom Fenech · Accepted Answer · 2015-10-02 12:49:20Z

4

It is wrong to assume that all flavours of regex are the same. In this case, \d is not supported by bash regular expressions. You should change your regex to this:

chk_regex='\.[0-9]+-[0-9]+-[0-9]+\.[0-9]+-[0-9]+-[0-9]+'

Of course, this assumes that when you say \d you don't require anything more than the digits from 0 to 9, as opposed to anything considered to be a digit in your locale. If you want to also match characters outside this range, then [[:digit:]] is probably what you want, instead of [0-9].

If you don't require parameter expansion, it's generally a good habit to use ' rather than ".

I have also removed the leading and trailing .* (as they don't do anything useful) and un-escaped the - (thanks for the comment gniourf_gniourf).

Working example:

$ file_name="xyz_abc_diagnostics.wifi2.2015-07-30.12-30-52.tar.gz"
$ chk_regex='\.[0-9]+-[0-9]+-[0-9]+\.[0-9]+-[0-9]+-[0-9]+'
$ if [[ "$file_name" =~ $chk_regex  ]];then
> echo "in obs regex"
> else
> echo "dont triggered"
> fi
in obs regex

As you can see, the pattern matches, so the if branch is taken.

As mentioned in the comments, you can use globs to match this pattern as well:

[[ $file_name = *.+([[:digit:]])-+([[:digit:]])-+([[:digit:]]).+([[:digit:]])-+([[:digit:]])-+([‌[:digit:]])* ]]

Granted, it's longer to write but globs may be useful if you wanted to loop through files matching this pattern, for example:

for archive in *.+([[:digit:]])-+([[:digit:]])-+([[:digit:]]).+([[:digit:]])-+([[:digit:]])-+([‌[:digit:]])*
do
    # some stuff
done

Note that in the example containing a loop (and in both examples on older versions of bash) you will need to enable extended globs using shopt -s extglob.

edited Oct 2, 2015 at 12:49

answered Oct 2, 2015 at 10:25

Tom Fenech

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4 Comments

gniourf_gniourf Over a year ago

Now thinking about it, the glob looks longer and awkward, but you can use a variable to hold some parts of it: ds=+([[:digit:]])-+([[:digit:]])-+([[:digit:]]) and then [[ $file_name = *.$ds.$ds* ]].

Tom Fenech Over a year ago

@gniourf_gniourf I was considering the same, although I guess at that point you're swapping one line for two. I'll leave the decision up to the OP :)

chepner Over a year ago

You also need shopt -s extglob in some versions of bash. I can never remember when that became the default inside [[ ... ]].

gniourf_gniourf Over a year ago

@chepner [[ ... ]] is parsed as an extended glob since 4.1.

Wiktor Stribiżew · Accepted Answer · 2015-10-02 10:39:29Z

2

Here is a fix, use [0-9] class instead of a \d and use {2} limiting quantifier to make it shorter (and really, the leading/trailing .* are useless since you are not using the matched string, just check for presence):

#!/bin/bash
file_name="xyz_abc_diagnostics.wifi2.2015-07-30.12-30-52.tar.gz"
chk_regex="(\.[0-9]+(-[0-9]+){2}){2}"
if [[ "$file_name" =~ $chk_regex  ]];then
echo "in obs regex"
else
echo "dont triggered"
fi

See IDEONE demo

Result: in obs regex

edited Oct 2, 2015 at 10:39

answered Oct 2, 2015 at 10:25

Wiktor Stribiżew

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2 Comments

Wiktor Stribiżew Over a year ago

@gniourf_gniourf: Thank you. I still wonder why IDEONE showed the right message when using $-prepended variable names.

gniourf_gniourf Over a year ago

Because both variables were unset (so expanded to the empty string), and the test boiled down to: [[ '' =~ '' ]] which is true. Try it: [[ "$nonexistent" =~ $nonexistent ]] && echo yes.

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