Update: Ignore this. I didn't look at the example (or comments) in the code.

Got a few more examples? 

> $in = '423RY75Y69827EC67592C78657N965R';;
> ( $out = $in ) =~ s[((\D)[0-9N]+\2)][_$1_]g;; 
> print $out;;
423R_Y75Y_69827E_C67592C_78657N965R

[download]

Update2: This appears to do it, but it may be dependant upon specifics of the single example? 

> $in = '423RY75Y69827EC67592C78657N965R345U299M';;
> ( $out = $in ) =~ s[ 
    ( ( \D ) [0-9N]+ \2 ) 
  | 
    ( [0-9N]+? [A-MO-Z] (?=\d) ) 
]{ defined $1 ? "_$1_" : "$3_" }egx; 

print $out;;
423R_Y75Y_69827E_C67592C_78657N965R_345U_299M

[download]
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re: match if last char and first char are different excluding certain chars (Updated) by BrowserUk
in thread match if last char and first char are different excluding certain chars by leocharre

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