Find the location with a match, then run a replace from that point on.

#!/usr/bin/perl
use strict;
use warnings;

while(<DATA>) {
    if( m{  \A  OBX  \|  [^|]*  \|  }gmsx ) {
        s{
            \G (?: ([[:upper:]]) | ([[:lower:]]) | ([^\^]) )
        }{
            defined $1 ? "U$1" :
            defined $2 ? "L$2" :
            $3
        }sexmg;
    }
    print;
}

__END__
NTE||L|obr note
OBX|NM|aaA..^Haem^RD2|7.5|g/dL|13.0-18.0|OR|
NTE||L|obx note 1/1 for 3058
OBX|NM|dBf..^TWC^RD2|8.9|10*9/L|4.0-11.0||

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Makeshifts last the longest.

In reply to Re: Can anyone make this regex job neater? by Aristotle
in thread Can anyone make this regex job neater? by Melly

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