my $greekReplace = "alpha|beta|chi|delta|epsilon|eta|gamma|hbar|kappa|lambda|mu|nu|omega|phi|pi|psi|rho|sigma|tau|theta";
my $string = '3*mu';
$string =~ s/($greekReplace)/\\$1 /isg;

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;

my $greekReplace = "alpha|beta|chi|delta|epsilon|eta|gamma|hbar|kappa|lambda|mu|nu|omega|phi|pi|psi|rho|sigma|tau|theta";

my %strings = ( 
    mu  => { valid => '\mu ' },
    amu => { valid => 'amu' },
    bmu => { valid => 'b\mu ' },
    cmu => { valid => 'c\mu ' },
    mud => { valid => '\mu d' },
    '3*mu' => { valid => '3*\mu ' },
);

foreach my $string (sort keys %strings) {
    print "Before: '$string'; ";
    my $string2 = $string;
    $string2 =~ s/($greekReplace)/\\$1 /isg;
    print "After: '$string2' ; Should be: $strings{$string}->{valid}; ";
    print $string2 eq $strings{$string}->{valid} ? "Passes\n" : "FAILS\n";
}
print "\n";