I have the 57-byte text file (in UTF-8). It can be generated by the following command:

echo '3C6469763E3C6469763E5F3C2F6469763E5F3C68313E6162636465665F3C2F68313E5F3C2F6469763E3C6469763EF09F93853C2F6469763E0A' | xxd -r -p > input.txt

I am trying to extract the part between _<h1> and _</h1>. I used the following program for gawk:

LC_ALL=en_US.utf8 gawk 'BEGIN {IGNORECASE = 1; } 
{match($0, /^.*_<h1>(.*)_</h1>.*$/, a); 
print(a[1]) > "output.txt"}' input.txt

Question: why is a[1] empty for the combination of this particular program and this particular input? If I remove the IGNORECASE = 1 part or replace /^.*_<h1>(.*)_</h1>.*$/ with /_<h1>(.*)_</h1>/, the output is correct (because a[1] contains abcdef). But how can any of these changes affect the output in my situation?