Regular Expression Period Issue

Question

((https?|ftp)\:\/\/|www.)(\S+[^.*])

I would like this expression to check for . in succession to each other. If it finds two or more periods back to back, the expression should fail. On the other hand, if it succeeds, I want it to match every character and/or symbol up until the first white space encountered.

In other words: www.yahoo..com should fail

On a related note: I realize that this expression is very basic in terms of judging valid URL structure. I have another "more intelligent" regular expression in place that precedes the one above. The purpose of the posted one is meant to check the validity of the URL that is passed from the initial regular expression via preg_match_all.

Answer 1

Using negative lookahead is an easy way if your engine supports it:

(?!.*\.\.)((https?|ftp)\:\/\/|www.)(\S+[^.*])

Otherwise, you have to be more specific:

^((https?|ftp)\:\/\/|www.)((\.[^.]|[^.\s])+[^.*])($|\s+)

Answer 2

You may awnt to check out FILTER_VALIDATE_URL with http://php.net/manual/en/book.filter.php instead of using Regex to validate your URLS.

Here's example usage:

$url = "http://www.example.com";

if(!filter_var($url, FILTER_VALIDATE_URL))
  {
  echo "URL is not valid";
  }
else
  {
  echo "URL is valid";
  }

Answer 3

You can do something like this:

((https?|ftp)\:\/\/|www.)((?:[\w\-]+\.)*[\w\-]+)

This will not yet check for valid URLs, even if you skip double dots. I'd advise not to use regex if the language you're using (PHP?) has other means of validating an URL.

The RFC states the following:

; URL schemeparts for ip based protocols:
ip-schemepart = "//" login [ "/" urlpath ]
login = [ user [ ":" password ] "@" ] hostport
hostport = host [ ":" port ]
host = hostname | hostnumber
hostname = *[ domainlabel "." ] toplabel
domainlabel = alphadigit | alphadigit *[ alphadigit | "-" ] alphadigit
toplabel = alpha | alpha *[ alphadigit | "-" ] alphadigit
alphadigit = alpha | digit
hostnumber = digits "." digits "." digits "." digits
port = digits
user = *[ uchar | ";" | "?" | "&" | "=" ]
password = *[ uchar | ";" | "?" | "&" | "=" ]
urlpath = *xchar ; depends on protocol see section 3.1

; HTTP
httpurl = "http://" hostport [ "/" hpath [ "?" search ]]
hpath = hsegment *[ "/" hsegment ]
hsegment = *[ uchar | ";" | ":" | "@" | "&" | "=" ]
search = *[ uchar | ";" | ":" | "@" | "&" | "=" ]

; Miscellaneous definitions
lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" |
"i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" |
"q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" |
"y" | "z"
hialpha = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
"J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
"S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
alpha = lowalpha | hialpha
digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
"8" | "9"
safe = "$" | "-" | "_" | "." | "+"
extra = "!" | "*" | "'" | "(" | ")" | ","
national = "{" | "}" | "|" | "\" | "^" | "~" | "[" | "]" | "`"
punctuation = "<" | ">" | "#" | "%" | <">
reserved = ";" | "/" | "?" | ":" | "@" | "&" | "="
hex = digit | "A" | "B" | "C" | "D" | "E" | "F" |
"a" | "b" | "c" | "d" | "e" | "f"
escape = "%" hex hex
unreserved = alpha | digit | safe | extra
uchar = unreserved | escape
xchar = unreserved | reserved | escape
digits = 1*digit