Author: Jim Hester
Date: 2026-03-27

# URL Validation

#### Jim Hester

#### 2026-03-27

Consider the task of correctly validating a URL. From that page two conclusions can be made.

1. Validating URLs require complex regular expressions.
2. Creating a correct regular expression is hard! (only 1 out of 13 regexs were valid for all cases).

Because of this one may be tempted to simply copy the best regex you can find (gist).

The problem with this is that while you can copy it now, what happens later when you find a case that is not handled correctly? Can you correctly interpret and modify this?

    "^(?:(?:http(?:s)?|ftp)://)(?:\\S+(?::(?:\\S)*)?@)?(?:(?:[a-z0-9\u00a1-\uffff](?:-)*)*(?:[a-z0-9\u00a1-\uffff])+)(?:\\.(?:[a-z0-9\u00a1-\uffff](?:-)*)*(?:[a-z0-9\u00a1-\uffff])+)*(?:\\.(?:[a-z0-9\u00a1-\uffff]){2,})(?::(?:\\d){2,5})?(?:/(?:\\S)*)?$"

However if you re-create the regex with `rex` it is much easier to understand and modify later if needed.

    library(rex)
    library(magrittr)

    valid_chars <- rex(except_some_of(".", "/", " ", "-"))

    re <- rex(
      start,

      # protocol identifier (optional) + //
      group(list("http", maybe("s")) %or% "ftp", "://"),

      # user:pass authentication (optional)
      maybe(non_spaces,
        maybe(":", zero_or_more(non_space)),
        "@"),

      #host name
      group(zero_or_more(valid_chars, zero_or_more("-")), one_or_more(valid_chars)),

      #domain name
      zero_or_more(".", zero_or_more(valid_chars, zero_or_more("-")), one_or_more(valid_chars)),

      #TLD identifier
      group(".", valid_chars %>% at_least(2)),

      # server port number (optional)
      maybe(":", digit %>% between(2, 5)),

      # resource path (optional)
      maybe("/", non_space %>% zero_or_more()),

      end
    )

We can then validate that it correctly identifies both good and bad URLs. (IP address validation removed)

    good <- c("http://foo.com/blah_blah",
      "http://foo.com/blah_blah/",
      "http://foo.com/blah_blah_(wikipedia)",
      "http://foo.com/blah_blah_(wikipedia)_(again)",
      "http://www.example.com/wpstyle/?p=364",
      "https://www.example.com/foo/?bar=baz&inga=42&quux",
      "http://✪df.ws/123",
      "http://userid:password@example.com:8080",
      "http://userid:password@example.com:8080/",
      "http://userid@example.com",
      "http://userid@example.com/",
      "http://userid@example.com:8080",
      "http://userid@example.com:8080/",
      "http://userid:password@example.com",
      "http://userid:password@example.com/",
      "http://➡.ws/䨹",
      "http://⌘.ws",
      "http://⌘.ws/",
      "http://foo.com/blah_(wikipedia)#cite-1",
      "http://foo.com/blah_(wikipedia)_blah#cite-1",
      "http://foo.com/unicode_(✪)_in_parens",
      "http://foo.com/(something)?after=parens",
      "http://☺.damowmow.com/",
      "http://code.google.com/events/#&product=browser",
      "http://j.mp",
      "ftp://foo.bar/baz",
      "http://foo.bar/?q=Test%20URL-encoded%20stuff",
      "http://مثال.إختبار",
      "http://例子.测试",
      "http://-.~_!$&'()*+,;=:%40:80%2f::::::@example.com",
      "http://1337.net",
      "http://a.b-c.de",
      "http://223.255.255.254")

    bad <- c(
      "http://",
      "http://.",
      "http://..",
      "http://../",
      "http://?",
      "http://??",
      "http://??/",
      "http://#",
      "http://##",
      "http://##/",
      "http://foo.bar?q=Spaces should be encoded",
      "//",
      "//a",
      "///a",
      "///",
      "http:///a",
      "foo.com",
      "rdar://1234",
      "h://test",
      "http:// shouldfail.com",
      ":// should fail",
      "http://foo.bar/foo(bar)baz quux",
      "ftps://foo.bar/",
      "http://-error-.invalid/",
      "http://-a.b.co",
      "http://a.b-.co",
      "http://0.0.0.0",
      "http://3628126748",
      "http://.www.foo.bar/",
      "http://www.foo.bar./",
      "http://.www.foo.bar./")

    all(grepl(re, good) == TRUE)

    ## [1] TRUE

    all(grepl(re, bad) == FALSE)

    ## [1] TRUE

You can now see the power and expressiveness of building regular expressions with `rex`!