Why does [a-z] asterisk match numbers?

Question

I have 3 directories at current path.

$ls
a_0db_data  a_clean_0db_data  a_clean_data
$ls a_*_data
a_0db_data:

a_clean_0db_data:

a_clean_data:

$ls a_[a-z]*_data
a_clean_0db_data:

a_clean_data:

I expected last ls command to match only a_clean_data. Why did it also match the one containing 0?

bash --version
GNU bash, version 4.2.24(1)-release (i686-pc-linux-gnu)

Answer 1

The [a-z] part isn't what matches the number; it's the *. You may be confusing shell globbing and regular expressions.

Tools like grep accept various flavours of regexes (basic by default, -E for extended, -P for Perl regex)

E.g. (-v inverts the match)

$ ls a_[a-z]*_data | grep -v "[0-9]"
a_clean_data

If you want to use a bash regex, here is an example on how to test if the variable $ref is an integer:

re='^[0-9]+$'
if ! [[ $ref =~ $re ]] ; then
  echo "error"
fi

Answer 2

So the problem is: why does a_[a-z]*_data match a_clean_0db_data?

This can be broken down into four parts:

• a_ matches the beginning of a_clean_0db_data, leaving clean_0db_data to be matched

• [a-z] matches any character in the range a-z (e.g. c), leaving lean_0db_data to be matched

• * matches any number of characters, e.g. lean_0db

• _data matches the trailing _data

In regular expressions, [a-z]* would mean any number of characters (including zero) in the range of a..z, but you are dealing with shell globbing, not with regular expressions.

If you want regular expressions, a few find implementations have a -regex predicate for that:

find . -maxdepth 1 -regex "^.*/a_[a-z]*_data$"

The -maxdepth is only here to limit the search-results to the folder you are in. The regular expression matches the entire filename, therefore I have added a ^.*/ to match the path-portion

Answer 3

* in shell patterns matches 0 or more characters. It's not to be confused with the * regular expression operator that means 0 or more of the preceding atom.

There is no equivalent of regexp * in basic shell patterns. However, various shells have extensions for that.

• ksh has *(something):

  ls a_*([a-z])_data
  

• you can have the same in bash with shopt -s extglob or zsh with setopt kshglob:

  shopt -s extglob
  ls a_*([a-z])_data
  

• In zsh with extendedglob enabled, # is equivalent to regexp *:

  setopt extendedglob
  ls a_[a-z]#_data
  

• In recent versions of ksh93, you can also use regular expressions in globs. Here with extended regular expressions:

  ls ~(E:a_[a-z]*_data)
  

Note that [a-z] matches different things depending on the current locale. It generally matches only the 26 a to z latin non-accented letters in the C locale. In other locales, it generally matches more, and doesn't always make sense. To match a letter in your locale, you may prefer [[:alpha:]].