G'day learner@perl,

Firstly, it looks like your initial problems have been pointed out be Happy-the-monk. Just adding a little more detail:

• K* means match zero or more K characters.
• . means match any character (except a newline).
• K.* means match a K character followed by zero or more non-newline characters.
• To match a single . you can escape it (\.) or put it in a character class ([.]). [I think the latter stands out more and makes the intent more obvious but either is fine.]

To match filenames starting with CHECK and ending with .txt, this would be fine:

/^CHECK.*[.]txt$/
[download]

To exclude those with A1 before the .txt, you can insert what's known as a zero-width negative look-behind assertion which looks like (?<!pattern). The name's quite a mouthful but don't be put off by that: it's just a name. Take a look at Look-Around Assertions in perlre - Extended Patterns for a more detailed description (as well as similar positve and look-ahead assertions).

So, we want to assert that the pattern matches if we look-behind .txt and don't find (negative) A1. We write that like (?<!A1) and place it between /^CHECK.* and [.]txt$/ where it effectively takes up no space (zero-width). That gives:

/^CHECK.*(?<!A1)[.]txt$/
[download]

You haven't made it clear whether there must be any characters between CHECK and .txt; if so, change zero or more non-newline characters (.*) to one or more non-newline characters (.+).

Here's my test:

$ perl -Mstrict -Mwarnings -E '
    my @test_filenames = qw{
        CHECK_CH.ABC12_A1.txt
        CHECK_CH.ABC12.txt
        HECK_CH.ABC12_A1.txt
        HECK_CH.ABC12.txt
        CHECK_CH.A1.txt
        CHECK_CH..txt
        CHECK.txt
        CHECKA1.txt
        CHECK.ABC12.txt
    };

    say "*** May be nothing between CHECK & .txt ***";
    for (@test_filenames) {
        next unless /^CHECK.*(?<!A1)[.]txt$/;
        say;
    }

    say "*** Must be something between CHECK & .txt ***";
    for (@test_filenames) {
        next unless /^CHECK.+(?<!A1)[.]txt$/;
        say;
    }
'
*** May be nothing between CHECK & .txt ***
CHECK_CH.ABC12.txt
CHECK_CH..txt
CHECK.txt
CHECK.ABC12.txt
*** Must be something between CHECK & .txt ***
CHECK_CH.ABC12.txt
CHECK_CH..txt
CHECK.ABC12.txt
[download]

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