bash extglob

jt1122 · 11-26-2023, 07:09 AM

from the bash extglob man page: "?(pattern-list) Matches zero or one occurrence of the given patterns"

Code:

touch a b; echo ?(a)
a

if ? matches 0 or 1 occurrences why isn't b also shown?

Thanks

jkirchner · 11-26-2023, 07:39 AM

In the parentheses it says "pattern list" and you gave it a pattern of "a" hence it returned "a". Try with ?(b) and you will get b.

pan64 · 11-26-2023, 09:15 AM

with other words:
?(a) means either empty string or a single "a", but "b" is none of them.
Try ?(a|b) and other patterns too.

jt1122 · 11-26-2023, 10:36 AM

"?(pattern-list) Matches zero or one occurrence of the given patterns"

The pattern "b" has 0 occurrences of "a" so based on the above it should have been returned,

Again, why isn't "b" returned?

lvm_ · 11-26-2023, 11:16 AM

Because glob expression matches the complete filename, not a substring. *?(a)* would match

jkirchner · 11-26-2023, 11:19 AM

Quote:

Originally Posted by jt1122

"?(pattern-list) Matches zero or one occurrence of the given patterns"

The pattern "b" has 0 occurrences of "a" so based on the above it should have been returned,

Again, why isn't "b" returned?

It matches against the pattern list, so there are zero return or a match, but if it "matches" it only returns matched value in the pattern. Of what purpose/value would there be if you have a list of the whole alphabet and passed it to a pattern to show only 'a' if it showed the full list anyway?

The description just says it matches against the pattern list.

pan64 · 11-26-2023, 11:35 AM

Quote:

Originally Posted by jt1122

"?(pattern-list) Matches zero or one occurrence of the given patterns"

The pattern "b" has 0 occurrences of "a" so based on the above it should have been returned,

Again, why isn't "b" returned?

because you are looking for a full match, not a partial match. The full string should match the pattern, and actually in your example there is no match.

jt1122 · 11-27-2023, 02:55 AM

why should I care it "searches" also empty strings but returns only hits with non-empty ones?. It seems that searching "0 occurrences" makes no difference so @() = ?() and +() = *()

MWE:

Code:

touch a b aa ab ba
echo @(a|b)
a b
echo ?(a|b)
a b

so searching 0 or 1 (?) occurrences is the same as 1 (@) occurrence.

Code:

echo *(a|b)
a aa ab b ba
echo +(a|b)
a aa ab b ba

again, searching 0 or more (*) is then same as 1 or more (+) occurrences.

pan64 · 11-27-2023, 03:14 AM

again, it is about full match, not a partial match.
with other words: in case of a matching like this: pattern ~ sample a successful match means the pattern will/must cover the whole sample, not only a part of it.
@(a) will not match as, ab, whatever and any other strings, just a and/or empty string.

This engine (extglob) cannot do partial match, just only full match, so if you want to look for "anything else" you need to add it explicitly to the pattern. This a *, so what you are looking for is: @(a)*
Try this (too) and you will see the difference.

MadeInGermany · 11-28-2023, 05:10 AM

Quote:

Originally Posted by jt1122

why should I care it "searches" also empty strings but returns only hits with non-empty ones?. It seems that searching "0 occurrences" makes no difference so @() = ?() and +() = *()

MWE:

Code:

touch a b aa ab ba
echo @(a|b)
a b
echo ?(a|b)
a b

so searching 0 or 1 (?) occurrences is the same as 1 (@) occurrence.

Code:

echo *(a|b)
a aa ab b ba
echo +(a|b)
a aa ab b ba

again, searching 0 or more (*) is then same as 1 or more (+) occurrences.

The zero-or-more matches would include an empty filename - but this is not allowed.
But the following makes the difference visible:

Code:

shopt -s extglob
for pat in "*(a|b)" "+(a|b)" "?(a|b)" "@(a|b)"
do
  for x in "" a b ab bb
  do
    case $x in
    ( $pat )
      echo "$pat : '$x' matches, case-esac"
    ;;
    esac
    if [[ $x == $pat ]]
    then
      echo "$pat : '$x' matches, [[ == ]]"
    fi
  done
done