Best option available to allow empty string with a Regex

Question

I have a Regex to validate a password: at least one uppercase, one lowercase, one number and one special character:

^(?=[^a-z]*[a-z])(?=[^A-Z]*[A-Z])(?=\D*\d)(?=.*[$@$!%*?&,;.:-_])[A-Za-z\d$@$!%*?&,;.:-_]+

But I also want to allow empty because I will validate it in another way.

So I tried to options:

^((?=[^a-z]*[a-z])(?=[^A-Z]*[A-Z])(?=\D*\d)(?=.*[$@$!%*?&,;.:-_])[A-Za-z\d$@$!%*?&,;.:-_]+)*$

NOTE: I wrapped the Regex in () and added *$ at the end;

Or the following:

^$|(?=[^a-z]*[a-z])(?=[^A-Z]*[A-Z])(?=\D*\d)(?=.*[$@$!%*?&,;.:-_])[A-Za-z\d$@$!%*?&,;.:-_]+

NOTE: Added $| at start ...

What is the best approach? One of these 2 or some other?

I tried your approach before but when I use ^[A-Za-z0-9]*|$ to allow only alphanumeric and empty this allows Spaces and I have no idea why — Miguel Moura
– Miguel Moura, Commented Apr 19, 2016 at 11:41
Use an optional non-capturing group around the whole pattern between the anchors ^(?:...)?$ — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Apr 19, 2016 at 11:43
^$|^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[$@$!%*?&,;.:_-])[\w$@$!%*?&,;.:-]+$ should work fine. — anubhava
– anubhava, Commented Apr 19, 2016 at 11:44
@WiktorStribiżew Do you mean: ^(?:(?=[^a-z]*[a-z])(?=[^A-Z]*[A-Z])(?=\D*\d)(?=.*[$@$!%*?&,;.:-])[A-Za-z\d$@$!%*?&,;.:-]+)?$ — Miguel Moura
– Miguel Moura, Commented Apr 19, 2016 at 11:53
@anubhava why the $ after the + at the end? Would $| at start be enough? — Miguel Moura
– Miguel Moura, Commented Apr 19, 2016 at 11:55

Wiktor Stribiżew · Accepted Answer · 2016-04-19 12:11:08Z

If you have a regex matching at least something, but not the whole string, you need to use an alternation with $:

^(?:(?=[^a-z]*[a-z])(?=[^A-Z]*[A-Z])(?=\D*\d)(?=.*[$@$!%*?&,;.:_-])[A-Za-z\d$@$!%*?&,;.:_-]+|$)
^^^^                                                                                        ^^^

See the regex demo. Look:

^ - start of string
(?: - An alternation group start
- (?=[^a-z]*[a-z])(?=[^A-Z]*[A-Z])(?=\D*\d)(?=.*[$@$!%*?&,;.:_-])[A-Za-z\d$@$!%*?&,;.:_-]+ - match this
- | - or...
- $ - end of string.
) - end of the alternation

If you had a pattern that matched the whole string (say, ^\d+\s+\w+$ to match strings like 12 Apr), and you would like to also match an empty string, you could just enclose with an optional pattern:

^(?:\d+\s+\w+)?$
 ^^^         ^^

Details:

^ - start of string
(?:\d+\s+\w+)? - one or zero sequences of 1+ digtis followed with 1+ whitespaces followed with 1+ alphanumerics/underscores
$ - end of string

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