What song is playing?
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iG`^na( na)*$
iM`na
m`^8$
&gt;Batman Theme,Neal Hefti$n&gt;Na Na Hey Hey Kiss Him Goodbye,Steam
m`^10$
&gt;Katamari Damacy,Yuu Miyake
m`^11$
&gt;Hey Jude,The Beatles
[0-9].+
&gt;Land Of 1000 Dances,Wilson Pickett
m`^[0-9]
&gt;Africa,Toto
&gt;
Song: 
,
$nArtist: 


How it works:

IgnoreCase flag + Grep mode flag + Regex ^na( na)*$. If the input is valid, print it as is. If not, print nothing.

iG`^na( na)*$


IgnoreCase flag + Match mode flag + Regex na. Count the "na"s and print the number.

iM`na


If the string is exactly "8", replace by the second line.

m`^8$
&gt;Batman Theme,Neal Hefti$n&gt;Na Na Hey Hey Kiss Him Goodbye,Steam


If the string is exactly "10", replace by the second line.

m`^10$
&gt;Katamari Damacy,Yuu Miyake


If the string is exactly "11", replace by the second line.

m`^11$
&gt;Hey Jude,The Beatles


If the string matches [0-9].+, replace by the second line. This is neither true for single digit numbers, 10 and 11 as they already have been repaced nor any of the above replacement strings.

[0-9].+
&gt;Land Of 1000 Dances,Wilson Pickett


If none of the above matched, the string still starts with a number. Default to Toto, Africa.

m`^[0-9]
&gt;Africa,Toto


Replace the placeholders &gt; and , by Song: and Artist:.

&gt;
Song: 
,
$nArtist: