Alphabet Pattern Matching
PHP, 502 Bytes

&lt;?$c=count($x=explode("\n",$_GET[p]));for($t=$u=$o="",$f="substr",$n=0;$n&lt;$g=$_GET["n"];$n++){if(2651==$l=strlen($s))$o.=$f($x[$n],0,$g).$f($x[$n],1-$g)."\n";elseif($l==1026)echo($t=$f($x[$n],0,$g)).$f(strrev($t),1)."\n";elseif($f($s,-1)=="Y")echo$f($x[$n],0,$g-$n).$f($x[$n],-$n,$n)."\n";elseif($l==376&amp;&amp;$f($s,-2,1)=="\n")echo$f($x[$n],0,$g-$n)."\n";elseif($l==726){$t.=$x[$n]."\n";$n+1==$g?:$u=$x[$n]."\n".$u;}else echo$f($x[$n]??"",0,$g)."\n";}if($o)echo$o.substr(strrev($o),2*$g+1);if($t)echo$t.$u;


Works with the string length of a pattern. A pattern has these conditions.
Letter at begin an end. CR are removed.

Expanded

foreach($p as$s){ # all patterns in an array
    $c=count($x=explode("\n",$s));
    for($t=$u=$o="",$f="substr",$n=0;$n&lt;$g=$_GET["n"];$n++){
    if(2651==$l=strlen($s))$o.=$f($x[$n],0,$g).$f($x[$n],1-$g)."\n";
# square pattern
    elseif($l==1026)echo($t=$f($x[$n],0,$g)).$f(strrev($t),1)."\n";
#alphabet triangle up down
    elseif($f($s,-1)=="Y")echo$f($x[$n],0,$g-$n).$f($x[$n],-$n,$n)."\n";
# Tabula recta
    elseif($l==376&amp;&amp;$f($s,-2,1)=="\n")echo$f($x[$n],0,$g-$n)."\n";
# two half triangle
    elseif($l==726){$t.=$x[$n]."\n";$n+1==$g?:$u=$x[$n]."\n".$u;}
#alphabet triangle left right
    else echo$f($x[$n]??"",0,$g)."\n";
# all other
    }
    if($o)echo$o.substr(strrev($o),2*$g+1);
    if($t)echo$t.$u;
}