How to write a regular expression pattern that is capable of validating URIs?

Question

How does one go about authoring a Regular Expression that matches against all strings that are valid URIs, while failing to match against all strings that are invalid URIs?

To be specific about what I am referencing when I say URI, I have added a link below for the most current URI RFC standard. It defines the entity that I want to validate using a regular expression.

I don't need it to be able to parse the URI. I just need a regular expression for validating.

The .Net Regular Expression Format is preferred. (.Net V1.1)

My Current Solution:

^([a-zA-Z0-9+.-]+):(//([a-zA-Z0-9-._~!$&'()*+,;=:]*)@)?([a-zA-Z0-9-._~!$&'()*+,;=]+)(:(\\d*))?(/?[a-zA-Z0-9-._~!$&'()*+,;=:/]+)?(\\?[a-zA-Z0-9-._~!$&'()*+,;=:/?@]+)?(#[a-zA-Z0-9-._~!$&'()*+,;=:/?@]+)?$(:(\\d*))?(/?[a-zA-Z0-9-._~!$&'()*+,;=:/]+)?(\?[a-zA-Z0-9-._~!$&'()*+,;=:/?@]+)?(\#[a-zA-Z0-9-._~!$&'()*+,;=:/?@]+)?$

Answer 1

Does Uri.IsWellFormedUriString work for you?

Answer 2

The URI specification says:

The following line is the regular expression for breaking-down a well-formed URI reference into its components.

^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?

(I guess that's the same regex as in the STD66 link given in another answer.)

But breaking-down is not validating. To correctly validate a URI, one would have to translate the BNF for URIs to a regex. While some BNFs cannot be expressed as regular expressions, I think with this one it could be done. But it shouldn't be done - it would be a huge mess. It's better to use a library function.

Answer 3

The best regex I came up with according to RFC 3986 (https://www.rfc-editor.org/rfc/rfc3986) was the following:

// named groups
/^(?<scheme>[a-z][a-z0-9+.-]+):(?<authority>\/\/(?<user>[^@]+@)?(?<host>[a-z0-9.\-_~]+)(?<port>:\d+)?)?(?<path>(?:[a-z0-9-._~]|%[a-f0-9]|[!$&'()*+,;=:@])+(?:\/(?:[a-z0-9-._~]|%[a-f0-9]|[!$&'()*+,;=:@])*)*|(?:\/(?:[a-z0-9-._~]|%[a-f0-9]|[!$&'()*+,;=:@])+)*)?(?<query>\?(?:[a-z0-9-._~]|%[a-f0-9]|[!$&'()*+,;=:@]|[/?])+)?(?<fragment>\#(?:[a-z0-9-._~]|%[a-f0-9]|[!$&'()*+,;=:@]|[/?])+)?$/i

// unnamed groups
/^([a-z][a-z0-9+.-]+):(\/\/([^@]+@)?([a-z0-9.\-_~]+)(:\d+)?)?((?:[a-z0-9-._~]|%[a-f0-9]|[!$&'()*+,;=:@])+(?:\/(?:[a-z0-9-._~]|%[a-f0-9]|[!$&'()*+,;=:@])*)*|(?:\/(?:[a-z0-9-._~]|%[a-f0-9]|[!$&'()*+,;=:@])+)*)?(\?(?:[a-z0-9-._~]|%[a-f0-9]|[!$&'()*+,;=:@]|[/?])+)?(\#(?:[a-z0-9-._~]|%[a-f0-9]|[!$&'()*+,;=:@]|[/?])+)?$/i

capture groups

1. scheme
2. authority
3. userinfo
4. host
5. port
6. path
7. query
8. fragment

Answer 4

The best and most definitive guide to this I have found is here: http://jmrware.com/articles/2009/uri_regexp/URI_regex.html (In answer to your question, see the URI table entry)

All of these rules from RFC3986 are reproduced in Table 2 along with a regular expression implementation for each rule.

A javascript implementation of this is available here: https://github.com/jhermsmeier/uri.regex

For reference, the URI regex is repeated below:

# RFC-3986 URI component:  URI
[A-Za-z][A-Za-z0-9+\-.]* :                                      # scheme ":"
(?: //                                                          # hier-part
  (?: (?:[A-Za-z0-9\-._~!$&'()*+,;=:]|%[0-9A-Fa-f]{2})* @)?
  (?:
    \[
    (?:
      (?:
        (?:                                                    (?:[0-9A-Fa-f]{1,4}:)    {6}
        |                                                   :: (?:[0-9A-Fa-f]{1,4}:)    {5}
        | (?:                            [0-9A-Fa-f]{1,4})? :: (?:[0-9A-Fa-f]{1,4}:)    {4}
        | (?: (?:[0-9A-Fa-f]{1,4}:){0,1} [0-9A-Fa-f]{1,4})? :: (?:[0-9A-Fa-f]{1,4}:)    {3}
        | (?: (?:[0-9A-Fa-f]{1,4}:){0,2} [0-9A-Fa-f]{1,4})? :: (?:[0-9A-Fa-f]{1,4}:)    {2}
        | (?: (?:[0-9A-Fa-f]{1,4}:){0,3} [0-9A-Fa-f]{1,4})? ::    [0-9A-Fa-f]{1,4}:
        | (?: (?:[0-9A-Fa-f]{1,4}:){0,4} [0-9A-Fa-f]{1,4})? ::
        ) (?:
            [0-9A-Fa-f]{1,4} : [0-9A-Fa-f]{1,4}
          | (?: (?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?) \.){3}
                (?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)
          )
      |   (?: (?:[0-9A-Fa-f]{1,4}:){0,5} [0-9A-Fa-f]{1,4})? ::    [0-9A-Fa-f]{1,4}
      |   (?: (?:[0-9A-Fa-f]{1,4}:){0,6} [0-9A-Fa-f]{1,4})? ::
      )
    | [Vv][0-9A-Fa-f]+\.[A-Za-z0-9\-._~!$&'()*+,;=:]+
    )
    \]
  | (?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}
       (?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)
  | (?:[A-Za-z0-9\-._~!$&'()*+,;=]|%[0-9A-Fa-f]{2})*
  )
  (?: : [0-9]* )?
  (?:/ (?:[A-Za-z0-9\-._~!$&'()*+,;=:@]|%[0-9A-Fa-f]{2})* )*
| /
  (?:    (?:[A-Za-z0-9\-._~!$&'()*+,;=:@]|%[0-9A-Fa-f]{2})+
    (?:/ (?:[A-Za-z0-9\-._~!$&'()*+,;=:@]|%[0-9A-Fa-f]{2})* )*
  )?
|        (?:[A-Za-z0-9\-._~!$&'()*+,;=:@]|%[0-9A-Fa-f]{2})+
    (?:/ (?:[A-Za-z0-9\-._~!$&'()*+,;=:@]|%[0-9A-Fa-f]{2})* )*
|
)
(?:\? (?:[A-Za-z0-9\-._~!$&'()*+,;=:@/?]|%[0-9A-Fa-f]{2})* )?   # [ "?" query ]
(?:\# (?:[A-Za-z0-9\-._~!$&'()*+,;=:@/?]|%[0-9A-Fa-f]{2})* )?   # [ "#" fragment ]

Answer 5

Are there some specific URIs you care about or are you trying to find a single regex that validates STD66?

I was going to point you to this regex for parsing a URI. You could then, in theory, check to see if all of the elements you care about are there.

But I think bdukes answer is better.