正则表达式创建允许，不允许几个字符

Question

I am new to regex, i have this use case:

1. Allow characters, numbers.
2. Zero or one question mark allowed. (? - valid, consecutive question marks are not allowed (??)).

test-valid

?test - valid

??test- invalid

?test?test - valid

???test-invalid

test??test -invalid

3. Exlcude $ sign.

[a-zA-Z0-9?] - seems this doesn't work

Thanks.

Answer 1

Try the following regular expression:
^(?!.*\\?\\?)[a-zA-Z0-9?]+$

1. first we're using Negetive lookahead - which allows us to exclude any character which is followed by double question marks ( Negetive lookahaed does not consume characters)

2. Since question mark has special meaning in regular expressions ( Quantifier — Matches between zero and one times), each question mark is escaped using backslash.

3. The plus sign at the end is a Quantifier — Matches between one and unlimited times, as many times as possible

You can test it here

Answer 2

Your description can be broken down into the regex:

^(?:\??[a-zA-Z0-9])+\??$

You say characters and your description shows letters and numbers only, but it's possible \\w (word characters) may be used instead - this includes underscore

It's between ^ and $ meaning the whole field must match (no partial matches, although if you want those you can remove this. The + means there must be at least one match (so empty string won't match). The capturing group ( (\\??[a-zA-Z0-9]) ) says I must either see a question mark followed by letters or just letters repeating many times, and the final question mark allows the string to end with a single question mark.

You probably don't want capturing groups here, so we can start that with ?: to prevent capture leading to:

^(?:\??[a-zA-Z0-9])+\??$

Matches

test  
?test  
?test?test  
test?

Doesn't match

??test
???test
test??test
test??
<empty string>
?