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/^d{1,}.?d?-?(d{1,})?.?d?$/The matching process is as follows:
First d{1,} matches "20";
.? matches ".";
d? matches "5", ? matches 1 time;
-? matches "", because ? matches 0 or 1 times, match here 0 times;
(d{1,})? matches "555", at this time ? matches 1 time;
.?d? matches "", at this time both ? match 0 times ;
$ matches the end of the string, so "20.5555" will match.

Update1: The matching process of
/^d{1,}.?d{1}?-?(d{1,})?.?d{1}?$/ is as follows:

1. d{1,}matches "20";

2. .?matches ".";

3. d{1}? will first try to match a number, and then match "5", ?match 1 time;

4. -? will match "", at this time ? matches 0 times;

5. (d{1,})?matches "555";

6. .? matches "", at this time ? matches 0 times;

7. d{1}? matches "", ? matches 0 times;
   d{1} means that the number is repeated once, so the regular expression is actually the same as d, so the updated regular expression There is no difference between the expression and the original regular expression.

Note: There was a problem with the matching process I wrote at the beginning, but it has been updated now.

const regex = /^\d+(?:\.\d)?(?:-\d+(?:\.\d)?)?$/;

const cases = [
    "1",
    "10",
    "100",
    "1-5",
    "10-50",
    "100-500",
    "0.5",
    "10.5",
    "10.5-20",
    "10.5-20.5",
    "10-20.5",
    "20.5555",
    "20.5-20.5555"
];

const r = cases.map(s => regex.test(s));
console.log(r);

???Why use regular expressions