Question

I am trying to match

Begin_Page {Some Number}

while trying to avoid

Begin_Page_{Some Number}

I tried

$line =~ m/^Begin_Page\s/

but I noticed it sometimes does not match anything file while surely had Begin_Page 8703

Answer 1

Your regexp is correct; the line should match:

while (<DATA>) {
    if (/^Begin_Page\s/) { print "OK: " }
    else                 { print "KO: " }
    print;
}

__DATA__
test
1233
Begin_Page 123
Begin_Page_456
Begin_Page 8703

Output:

KO: test
KO: 1233
OK: Begin_Page 123
KO: Begin_Page_456
OK: Begin_Page 8703