Difference between `${parameter#word}` and `${parameter##word}`?

Question

According to man bash:

${parameter#word}  
${parameter##word}
    Remove matching prefix pattern. The word is expanded to produce a
    pattern just as in pathname expansion, and matched against the
    expanded value of parameter using the rules described under Pattern
    Matching below. If the pattern matches the beginning of the value of
    parameter, then the result of the expansion is the expanded value of
    parameter with the shortest matching pattern (the ``#'' case) or the
    longest matching pattern (the ``##'' case) deleted. If parameter is
    @ or *, the pattern removal operation is applied to each positional
    parameter in turn, and the expansion is the resultant list.  If
    parameter is an array variable subscripted with @ or *, the pattern
    removal operation is applied to each member of the array in turn,
    and the expansion is the resultant list.

Somehow, I don't manage to find a situation that gives different results between ${parameter#word} and ${parameter##word}.

I am looking for situations that would illustrate the different behaviours between the two syntaxes. And incidentally, what would be a shortest matching pattern and what would be a longest matching pattern?

Answer 1

This example may be more illustrative

$ parameter="This is a sentence with many words. Some of the words appear more than once. Oh, my word!"
$ echo "${parameter#*word}"
s. Some of the words appear more than once. Oh, my word!
$ echo "${parameter##*word}"
!

So:

• ${parameter#pattern} removes the shortest match of the pattern
• ${parameter##pattern} removes the longest match of the pattern

There are also ${parameter%pattern} and ${parameter%pattern} that match the pattern at the end of the string. I remember the difference because # appears before % on a (US) keyboard.

Answer 2

I just found this example:

$ INT=00011
$ echo "${INT#*0}"
0011
$ echo "${INT##*0}"
11

Here, we first assign the string "00011" to the variable 'INT', then we output the result of the parameter expansion performed by either ${parameter#word} or ${parameter##word} with 'INT' as the parameter and '*0' as the word pattern.

The minimal [prefix] match of '*0' with the parameter is just '0', whereas the maximal [prefix] match is '000'.