正则表达式：<!--?--> ?=?\d{4}：它匹配什么？

Question

In a C# class, I came across this regular expression:

<?>?=?\d{4}  

It is pretty obvious that its last part ( \d{4} ) matches 4 decimal digits but what about <?>?=? ? What does it match?

Thanks for any explanations.

Answer 1

Four digits at the end preceded by the < , > and = occurring zero or once in that order.

Match:

<>=1234
>=1234
=1234
1234
<=1234

Answer 2

The expression '<?>?=?' matches a '<' char (or none) possibly followed by a '>' possibly followed by a '='. Thus all of the following will match:

1. ''
2. '<'
3. '>'
4. '='
5. '<>'
6. '<='
7. '>='
8. '<>='

Answer 3

The question mark after the characters make it optional, so it matches any combination where each character can be present or not:

• <>=
• <>
• <=
• <
• >=
• >
• =

It's probably meant to match any of the three characters on its own, but then you would rather use [<>=]? instead.