A is a logical array with the same number of rows as the string array readfile. So if you remove/ignore the first element of A, it uses the logical value for row 2 to determine whether or not to select row 1, the logical value for row 3 to determine whether or not to select row 2, etc.
readfile = readlines("sampleText.txt")
pat = "X";
A = contains(readfile,pat)
linesContainingX = readfile(A)
linesBeforeX = readfile(A(2:end))
Note that if the first line of the file matches the pattern, starting your indexing using element 2 of A will skip that line in linesBeforeX; but then again, there isn't a "line before" the first line.
linesContainingTest = contains(readfile, "Test")
linesBeforeContainingTest = readfile(linesContainingTest(2:end))
