Regular expression to match DNS hostname or IP Address

Question

Does anyone have a regular expression handy that will match any legal DNS hostname or IP address   It s easy to write one that works 95  of the time  but I m hoping to get something that s well tested to exactly match the latest RFC specs for DNS hostnames

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def isValidHostname hostname        if len hostname   gt  255          return False     if hostname -1                   hostname   hostname  -1      strip exactly one dot from the right                                        if present     allowed   re compile     -  A-Z d-  1 63    lt  -     re IGNORECASE      return all allowed match x  for x in hostname split

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I think this is the best Ip validation regex  please check it once           01   0-9   0-9  2  0-4  0-9  5 0-5       3   01   0-9   0-9  2  0-4  0-9  5 0-5

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The new Network framework has failable initializers for struct IPv4Address and struct IPv6Address which handle the IP address portion very easily  Doing this in IPv6 with a regex is tough with all the shortening rules   Unfortunately I don t have an elegant answer for hostname   Note that Network framework is recent  so it may force you to compile for recent OS versions   import Network let tests     192 168 4 4   fkjhwojfw   192 168 4 4 4   2620 3   2620  33    for test in tests       if let     IPv4Address test            debugPrint    test  is valid ipv4 address         else if let     IPv6Address test            debugPrint    test  is valid ipv6 address         else           debugPrint    test  is not a valid IP address            output   192 168 4 4 is valid ipv4 address   fkjhwojfw is not a valid IP address   192 168 4 4 4 is not a valid IP address   2620 3 is not a valid IP address   2620  33 is valid ipv6 address

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Here is a regex that I used in Ant to obtain a proxy host IP or hostname out of ANT OPTS  This was used to obtain the proxy IP so that I could run an Ant  isreachable  test before configuring a proxy for a forked JVM       -Dhttp  proxyHost   w 1     w 1     w 1      w 0    s

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AddressRegex      ftp http https        0-9  1 3    0-9  1 3    0-9  1 3    0-9  1 3   0-9  1 5       HostnameRegex       ftp http https        a-z0-9       a-z0-9  a-z0-9-       a-z  2 6       a-z  2 6      a-z  2 6     i   this re are used only for for this type validation  work only if http   www kk com http   www kk co in  not works for  http   www kk com  http   www kk co in kk  http   www kk com dfas http   www kk co in

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how about this     0-9  1 3     3  0-9  1 3

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Regarding IP addresses  it appears that there is some debate on whether to include leading zeros   It was once the common practice and is generally accepted  so I would argue that they should be flagged as valid regardless of the current preference   There is also some ambiguity over whether text before and after the string should be validated and  again  I think it should   1 2 3 4 is a valid IP but 1 2 3 4 5 is not and neither the 1 2 3 4 portion nor the 2 3 4 5 portion should result in a match   Some of the concerns can be handled with this expression   grep -E         alnum       0-1   0-9  1 2  2 0-4  0-9  25 0-5      3   0-1   0-9  1 2  2 0-4  0-9  25 0-5       alnum           The unfortunate part here is the fact that the regex portion that validates an octet is repeated as is true in many offered solutions   Although this is better than for instances of the pattern  the repetition can be eliminated entirely if subroutines are supported in the regex being used   The next example enables those functions with the -P switch of grep and also takes advantage of lookahead and lookbehind functionality   The function name I selected is  o  for octet   I could have used  octet  as the name but wanted to be terse    grep -P     lt    d w       lt o gt   0-1   0-9  1 2  2 0-4  0-9  25 0-5       g lt o gt   3      d w        The handling of the dot might actually create a false negatives if IP addresses are in a file with text in the form of sentences since the a period could follow without it being part of the dotted notation   A variant of the above would fix that   grep -P     lt    d w       lt x gt   0-1   0-9  1 2  2 0-4  0-9  25 0-5       g lt x gt   3       d w     d

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To match a valid IP address use the following regex    25 0-5  2 0-4  0-9   01   0-9  0-9       25 0-5  2 0-4  0-9   01   0-9  0-9     3    instead of     01   0-9  0-9   2 0-4  0-9  25 0-5       01   0-9  0-9   2 0-4  0-9  25 0-5    3    Explanation  Many regex engine match the first possibility in the OR sequence  For instance  try the following regex    10 48 0 200   Test  Test the difference between good vs bad

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I thought about this simple regex matching pattern for IP address matching  d     d     d     d

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on php  filter var gethostbyname  dns   FILTER VALIDATE IP     true    ip     not ip

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I found this works pretty well for IP addresses  It validates like the top answer but it also makes sure the ip is isolated so no text or more numbers decimals are after or before the ip                      lt   S        d  1-9  d 1 d d 2 0-4  d 25 0-5   b   b  7     S

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It s worth noting that there are libraries for most languages that do this for you  often built into the standard library  And those libraries are likely to get updated a lot more often than code that you copied off a Stack Overflow answer four years ago and forgot about  And of course they ll also generally parse the address into some usable form  rather than just giving you a match with a bunch of groups   For example  detecting and parsing IPv4 in  POSIX  C    include  lt arpa inet h gt   include  lt stdio h gt   int main int argc  char  argv        for  int i 1  i  argc    i        struct in addr addr    0       printf   s     argv i        if  inet pton AF INET  argv i    amp addr     1        printf  invalid n        else       printf   u n   addr s addr         return 0        Obviously  such functions won t work if you re trying to  e g   find all valid addresses in a chat message   but even there  it may be easier to use a simple but overzealous regex to find potential matches  and then use the library to parse them   For example  in Python    gt  gt  gt  import ipaddress  gt  gt  gt  import re  gt  gt  gt  msg    My address is 192 168 0 42  192 168 0 420 is not an address   gt  gt  gt  for maybeip in re findall r  d 1 3    d 1 3    d 1 3    d 1 3    msg           try              print ipaddress ip address maybeip           except ValueError              pass

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This works for valid IP addresses   regex       0-9   1-9  0-9   1  0-9  0-9   2  0-5  0-5       0-9   1-9  0-9   1  0-9  0-9   2  0-5  0-5       0-9   1-9  0-9   1  0-9  0-9   2  0-5  0-5       0-9   1-9  0-9   1  0-9  0-9   2  0-5  0-5

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The hostname regex of smink does not observe the limitation on the length of individual labels within a hostname  Each label within a valid hostname may be no more than 63 octets long   ValidHostnameRegex     a-zA-Z0-9   a-zA-Z0-9  a-zA-Z0-9 -  0 61  a-zA-Z0-9         a-zA-Z0-9   a-zA-Z0-9  a-zA-Z0-9 -  0 61  a-zA-Z0-9        Note that the backslash at the end of the first line  above  is Unix shell syntax for splitting the long line  It s not a part of the regular expression itself   Here s just the regular expression alone on a single line      a-zA-Z0-9   a-zA-Z0-9  a-zA-Z0-9 -  0 61  a-zA-Z0-9       a-zA-Z0-9   a-zA-Z0-9  a-zA-Z0-9 -  0 61  a-zA-Z0-9       You should also check separately that the total length of the hostname must not exceed 255 characters  For more information  please consult RFC-952 and RFC-1123

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You can use the following regular expressions separately or by combining them in a joint OR expression   ValidIpAddressRegex        0-9   1-9  0-9  1 0-9  2  2 0-4  0-9  25 0-5      3   0-9   1-9  0-9  1 0-9  2  2 0-4  0-9  25 0-5       ValidHostnameRegex        a-zA-Z0-9   a-zA-Z0-9  a-zA-Z0-9 -   a-zA-Z0-9        A-Za-z0-9   A-Za-z0-9  A-Za-z0-9 -   A-Za-z0-9        ValidIpAddressRegex matches valid IP addresses and ValidHostnameRegex valid host names  Depending on the language you use   could have to be escaped with       ValidHostnameRegex is valid as per RFC 1123  Originally  RFC 952 specified that hostname segments could not start with a digit   http   en wikipedia org wiki Hostname     The original specification of   hostnames in RFC   952    mandated that labels could not start   with a digit or with a hyphen  and   must not end with a hyphen  However  a   subsequent specification  RFC   1123    permitted hostname labels to start   with digits    Valid952HostnameRegex        a-zA-Z   a-zA-Z  a-zA-Z0-9 -   a-zA-Z0-9        A-Za-z   A-Za-z  A-Za-z0-9 -   A-Za-z0-9

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d 1 2  1  d 2  2 0-4   d 25 0-5      3    d 1 2  1  d 2  2 0-4   d 25 0-5

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Checking for host names like    mywebsite co in  thangaraj name  18thangaraj in  thangaraj106 in etc     a-z d         w 2 4

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a-zA-Z0-9    a-zA-Z0-9  -a-zA-Z0-9   a-zA-Z0-9        a-zA-Z0-9    a-zA-Z0-9  -a-zA-Z0-9   a-zA-Z0-9

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I don t seem to be able to edit the top post  so I ll add my answer here   For hostname - easy answer  on egrep example here -- http    www linuxinsight com how to grep for ip addresses using the gnu egrep utility html  egrep      digit    1 3     3    digit    1 3     Though the case doesn t account for values like 0 in the fist octet  and values greater than 254  ip addres  or 255  netmask   Maybe an additional if statement would help   As for legal dns hostname  provided that you are checking for internet hostnames only  and not intranet   I wrote the following snipped  a mix of shell php but it should be applicable as any regular expression   first go to ietf website  download and parse a list of legal level 1 domain names   tld   curl -s http   data iana org TLD tlds-alpha-by-domain txt    sed 1d    cut -f1 -d -    tr   n        sed  s          1    echo    tld     That should give you a nice piece of re code that checks for legality of top domain name  like  com  org or  ca  Then add first part of the expression according to guidelines found here -- http    www domainit com support faq mhtml category Domain FAQ amp question 9  any alphanumeric combination and  -  symbol  dash should not be in the beginning or end of an octet      a-z0-9     a-z0-9   -   a-z0-9            Then put it all together  PHP preg match example     pattern         a-z0-9     a-z0-9   -   a-z0-9          AC AD AE AERO AF AG AI AL AM AN AO AQ AR ARPA AS ASIA AT AU AW AX AZ BA BB BD BE BF BG BH BI BIZ BJ BM BN BO BR BS BT BV BW BY BZ CA CAT CC CD CF CG CH CI CK CL CM CN CO COM COOP CR CU CV CX CY CZ DE DJ DK DM DO DZ EC EDU EE EG ER ES ET EU FI FJ FK FM FO FR GA GB GD GE GF GG GH GI GL GM GN GOV GP GQ GR GS GT GU GW GY HK HM HN HR HT HU ID IE IL IM IN INFO INT IO IQ IR IS IT JE JM JO JOBS JP KE KG KH KI KM KN KP KR KW KY KZ LA LB LC LI LK LR LS LT LU LV LY MA MC MD ME MG MH MIL MK ML MM MN MO MOBI MP MQ MR MS MT MU MUSEUM MV MW MX MY MZ NA NAME NC NE NET NF NG NI NL NO NP NR NU NZ OM ORG PA PE PF PG PH PK PL PM PN PR PRO PS PT PW PY QA RE RO RS RU RW SA SB SC SD SE SG SH SI SJ SK SL SM SN SO SR ST SU SV SY SZ TC TD TEL TF TG TH TJ TK TL TM TN TO TP TR TRAVEL TT TV TW TZ UA UG UK US UY UZ VA VC VE VG VI VN VU WF WS XN XN XN XN XN XN XN XN XN XN XN YE YT YU ZA ZM ZW       i        if  preg match   pattern   matching string           do stuff         You may also want to add an if statement to check that string that you checking is shorter than 256 characters -- http   www ops ietf org lists namedroppers namedroppers 2003 msg00964 html

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gt  gt  gt  my hostname    testhostn ame   gt  gt  gt  print bool re match      a-zA-Z   a-zA-Z  a-zA-Z0-9 -   a-zA-Z0-9        A-Za-z   A-Za-z  A-Za-z0-9 -   A-Za-z0-9      my hostname   True  gt  gt  gt  my hostname    testhostn    ame   gt  gt  gt  print bool re match      a-zA-Z   a-zA-Z  a-zA-Z0-9 -   a-zA-Z0-9        A-Za-z   A-Za-z  A-Za-z0-9 -   A-Za-z0-9      my hostname   False  gt  gt  gt  my hostname    testhostn A ame   gt  gt  gt  print bool re match      a-zA-Z   a-zA-Z  a-zA-Z0-9 -   a-zA-Z0-9        A-Za-z   A-Za-z  A-Za-z0-9 -   A-Za-z0-9      my hostname   True

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try this     2 0-4  d 25 0-5   01   d d      3  2 0-4  d 25 0-5   01   d d     it works in my case

[regex] Regular expression to match DNS hostname or IP Address?

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