Regex to match one character exactly one time anywhere inside a string

Question

I want to match the symbol @ inside a string. It doesn't matter if it is at the beginning, at the middle or at the end of string or anywhere inside it. What matters is to just match it only 0 or 1 times.

I thought of something like this:

^[^@]*@[^@]*$

But definitely doesn't work because even though it matches the symbol @ once, it can match it only inside the string.

@foobar Valid
foo@bar Valid
foobar@ Valid
fooba@r Valid

but

@@foobar Invalid
@foo@bar Invalid
foo@b@ar Invalid

Shot in the dark here, but are you trying to validate an email address? — Niet the Dark Absol, Jan 21 '18 at 11:18
Possible duplicate of [regular expression to match a character that can only appear once in a string?](https://stackoverflow.com/questions/17869299/regular-expression-to-match-a-character-that-can-only-appear-once-in-a-string) — mickmackusa, Jan 22 '18 at 00:13

Allan · Accepted Answer · 2018-01-21T11:13:02.727

You can use the following regex:

^[^@]*@[^@]*$

to forbid to have more than one @ in your input string.

Tested:

https://regex101.com/r/H4hXF5/1

if you need to match also string without the @ in it add a ? just after it in the regex:

^[^@]*@?[^@]*$

this will also match string abc for example

explanations:

^ beginning of line
[^@]* then all char except the @ taken 0 to N times
@ then one @ char
[^@]* same as step2
$ ending of the line
? to have the character before it become optional

Regex to match one character exactly one time anywhere inside a string

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