(-?[.\d]+(?!<-)) should help

$stra = "52-52.5";
$strb = "25.3-26.5";
$strc = "25.3--26.5";
$strd = "-25.3-26.5";
$stre = "-25.3--26.5";

($str1) = $stra =~ /(-?[.\d]+(?!<-))/;
print $str1, $/;

($str1) = $strb =~ /(-?[.\d]+(?!<-))/;
print $str1, $/;

($str1) = $strc =~ /(-?[.\d]+(?!<-))/;
print $str1, $/;

($str1) = $strd =~ /(-?[.\d]+(?!<-))/;
print $str1, $/;

($str1) = $stre =~ /(-?[.\d]+(?!<-))/;
print $str1, $/;
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Update: added examples