testing for a fixed number of digits in bash?

Question

I have a need in a bash script to see if a string ends in an underscore and a date, of the form:

   _yyyymmdd

I don't care about validating the date at this point, just that I have something like

 abc_def_yyyymmdd

or

 abc_yyyymmdd

(Note there may be other underscores in the name that I don't care about; I only care that it ends in _yyyymmdd -- an underscore followed by 8 digits).

Can you assist me in that pattern match test? Let me be clear based on a comment that I don't care about validating the date at this point, just that the string (a filename) ends in an underscore and 8 digits.

what about a number of digits? I don't want to test for a valid date at this point. — Ray, Aug 20 '15 at 17:46
possible duplicate of [Reference - What does this regex mean?](http://stackoverflow.com/questions/22937618/reference-what-does-this-regex-mean) — Andy Brown, Aug 22 '15 at 13:39

Avinash Raj · Answer 1 · 2015-08-20T18:00:36.783

4

You may use the below regex,

[[ $str =~ _[0-9]{8}$ ]]

[0-9]{8} would match exactly 8 digit charcaters. $ asserts that we are at the end.

edited Aug 20 '15 at 18:00

answered Aug 20 '15 at 17:47

Avinash Raj

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The leading `.*` isn't necessary – chepner Aug 20 '15 at 17:59
I'm updating my cygwin and cannot test at the moment, can you tell me why @chepner? – Ray Aug 20 '15 at 18:00
The regular expression isn't anchored to the beginning of the string, so the match can start anyway. – chepner Aug 20 '15 at 18:00
Thanks for the insight. – Ray Aug 20 '15 at 18:01
oh just saw \d is not recognized by bash. – Ray Aug 20 '15 at 18:01
Any thought on @tripleee's comment -- at the bottom? – Ray Aug 20 '15 at 18:03

score 2 · Answer 2 · answered Aug 20 '15 at 17:53

2

You don't need regex.

case $filename in
  *_[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9] )
    echo "fine";;
  *)
    echo "fail";;
esac

answered Aug 20 '15 at 17:53

tripleee

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Thanks. What do you see as an advantage to no-regex vs. using regex? – Ray Aug 20 '15 at 17:55
Globs are simpler, computationally less expensive, and often (though not in this case) easier to read and debug. Additionally, this `case` construct is portable all the way back to Unix v7 and the original Bourne shell. – tripleee Aug 20 '15 at 17:59
It's a lot more concise, though, when it's available. If POSIX compatibility is needed, you can use `expr "$filename" : '.*_[0-9]\{8\}$'` (at the expense of starting a new process, of course.) – chepner Aug 20 '15 at 18:04

testing for a fixed number of digits in bash?

2 Answers2