I have the following string:
i:0#.w|domain\x123456
I know about the possibility to group searchterms by using <mysearchterm> and calling it via RegEx.Match(myRegEx).Result("${mysearchtermin}");.
I also know that I can lookbehind assertions like (?<= subexpression) via MSDN. Could someone help me in geting the (including the possibility to search for them via groups as shown before):
domain ("domain")
user account ("x12345")
I don't need anything from before the pipe character (nor the pipe character itself) - so basically I am interested in domain\x123456.
As others have noted, this can be done without regex, or without lookbehinds. That being said, I can think of reasons you might want them: to write a RegexValidator instead of having to roll up a CustomValidator, for example. In ASP.NET, CustomValidators can be a little longer to write, and sometimes a RegexValidator does the job just fine.
As far as lookbehinds, the main reason you'd want one for something like this is if the target string could contain irrelevant copies of the |domain\x123456 pattern:
foo#bar|domain\x999999 says: 'i:0#.w|domain\x888888i:0#.w|domain\x123456|domain\x000000'
If you only wanted to grab domain\x888888 and domain\x123456 out of that, a lookbehind could be useful. Or maybe you just want to learn about lookbehinds. Anyway, since we only have one sample input, I can only guess at the rules; so perhaps something like this:
#"(?<=[a-z]:\d#\.[a-z]\|)(?<domain>[^\\]*)\\(?<user>x\d+)"
Lookarounds are one of the most subtle and misunderstood features of regex, IMHO. I've gotten a lot of use out of them in preventing false positives, or in limiting the length of matches when I'm not trying to match the entire string (for example, if I want only the 3-digit numbers in blah 1234 123 1234567 123 foo, I can use (?<!\d)\d{3}(?!\d)). Here's a good reference if you want to learn more about named groups and lookarounds.
You can just use the regex #"\|([^\\]+)\\(.+)".
The domain and user will be in groups 1 and 2, respectively.
You don't need regular expressions for that.
var myString = #"i:0#.w|domain\x123456";
var relevantParts = myString.Split('|')[1].Split('\\');
var domain = relevantParts[0];
var user = relevantParts[1];
Explanation: String.Split(separator) returns an array of substrings separated by separator.
If you insist of using regular expressions, this is how you do it with named groups and Match.Result, based on SLaks answer (+1, by the way):
var myString = #"i:0#.w|domain\x123456";
var r = new Regex(#"\|(?<domain>[^\\]+)\\(?<user>.+)");
var match = r.Matches(myString)[0]; // get first match
var domain = match.Result("${domain}");
var user = match.Result("${user}");
Personally, however, I would prefer the following syntax, if you are just extracting the values:
var domain = match.Groups["domain"];
var user = match.Groups["user"];
And you really don't need lookbehind assertions here.
Related
I have an regex looking like this "[a-æøåA-ÆØÅ0-9-/().\s]{1,100}$". I would like to allow ONE / in user input from textbox, e.g. like "3/4 inch fitting bla bla".
How can I do that in a safe way, and is it safe at all?
My query look something like this.
XmlElement bemærkning = xmldoc.CreateElement("Bemærkning");
bemærkning.InnerText = txtBemærkningWT.Text;
//XmlElement usernamePCxml = xmldoc.CreateElement("UsernamepcXML");
//usernamePCxml.InnerText = usernamePC.ToString();
parentelement.AppendChild(type);
parentelement.AppendChild(art);
parentelement.AppendChild(l);
parentelement.AppendChild(bemærkning);
parentelement.AppendChild(varenummer);
parentelement.AppendChild(opretter);
parentelement.AppendChild(date);
//parentelement.AppendChild(usernamePCxml);
xmldoc.DocumentElement.AppendChild(parentelement);
xmldoc.Save(Server.MapPath(map));
Since you also check the total number of characters {1,100} there is no simple solution (that I can think of) for one regex. The easyest way is probably to do a separate check either for the occurence of / or for the overall length. If you check the total length separately you could use a regex like this:
"^[a-æøåA-ÆØÅ0-9-().\s]*\/?[a-æøåA-ÆØÅ0-9-().\s]*$"
Notice that I added a ^ at the beginnin to indicate the start of the input. I don'nt know if this is necessary in your case but it probably is.
Regarding the safity: you are using InnerText, which escapes markup that might be contained in the input, InnerXml does not. So you should be fine.
Say I have a regex matching a hexadecimal 32 bit number:
([0-9a-fA-F]{1,8})
When I construct a regex where I need to match this multiple times, e.g.
(?<from>[0-9a-fA-F]{1,8})\s*:\s*(?<to>[0-9a-fA-F]{1,8})
Do I have to repeat the subexpression definition every time, or is there a way to "name and reuse" it?
I'd imagine something like (warning, invented syntax!)
(?<from>{hexnum=[0-9a-fA-F]{1,8}})\s*:\s*(?<to>{=hexnum})
where hexnum= would define the subexpression "hexnum", and {=hexnum} would reuse it.
Since I already learnt it matters: I'm using .NET's System.Text.RegularExpressions.Regex, but a general answer would be interesting, too.
RegEx Subroutines
When you want to use a sub-expression multiple times without rewriting it, you can group it then call it as a subroutine. Subroutines may be called by name, index, or relative position.
Subroutines are supported by PCRE, Perl, Ruby, PHP, Delphi, R, and others. Unfortunately, the .NET Framework is lacking, but there are some PCRE libraries for .NET that you can use instead (such as https://github.com/ltrzesniewski/pcre-net).
Syntax
Here's how subroutines work: let's say you have a sub-expression [abc] that you want to repeat three times in a row.
Standard RegEx
Any: [abc][abc][abc]
Subroutine, by Name
Perl: (?'name'[abc])(?&name)(?&name)
PCRE: (?P<name>[abc])(?P>name)(?P>name)
Ruby: (?<name>[abc])\g<name>\g<name>
Subroutine, by Index
Perl/PCRE: ([abc])(?1)(?1)
Ruby: ([abc])\g<1>\g<1>
Subroutine, by Relative Position
Perl: ([abc])(?-1)(?-1)
PCRE: ([abc])(?-1)(?-1)
Ruby: ([abc])\g<-1>\g<-1>
Subroutine, Predefined
This defines a subroutine without executing it.
Perl/PCRE: (?(DEFINE)(?'name'[abc]))(?P>name)(?P>name)(?P>name)
Examples
Matches a valid IPv4 address string, from 0.0.0.0 to 255.255.255.255:
((?:25[0-5])|(?:2[0-4][0-9])|(?:[0-1]?[0-9]?[0-9]))\.(?1)\.(?1)\.(?1)
Without subroutines:
((?:25[0-5])|(?:2[0-4][0-9])|(?:[0-1]?[0-9]?[0-9]))\.((?:25[0-5])|(?:2[0-4][0-9])|(?:[0-1]?[0-9]?[0-9]))\.((?:25[0-5])|(?:2[0-4][0-9])|(?:[0-1]?[0-9]?[0-9]))\.((?:25[0-5])|(?:2[0-4][0-9])|(?:[0-1]?[0-9]?[0-9]))
And to solve the original posted problem:
(?<from>(?P<hexnum>[0-9a-fA-F]{1,8}))\s*:\s*(?<to>(?P>hexnum))
More Info
http://regular-expressions.info/subroutine.html
http://regex101.com/
Why not do something like this, not really shorter but a bit more maintainable.
String.Format("(?<from>{0})\s*:\s*(?<to>{0})", "[0-9a-zA-Z]{1,8}");
If you want more self documenting code i would assign the number regex string to a properly named const variable.
.NET regex does not support pattern recursion, and if you can use (?<from>(?<hex>[0-9a-fA-F]{1,8}))\s*:\s*(?<to>(\g<hex>)) in Ruby and PHP/PCRE (where hex is a "technical" named capturing group whose name should not occur in the main pattern), in .NET, you may just define the block(s) as separate variables, and then use them to build a dynamic pattern.
Starting with C#6, you may use an interpolated string literal that looks very much like a PCRE/Onigmo subpattern recursion, but is actually cleaner and has no potential bottleneck when the group is named identically to the "technical" capturing group:
C# demo:
using System;
using System.Text.RegularExpressions;
public class Test
{
public static void Main()
{
var block = "[0-9a-fA-F]{1,8}";
var pattern = $#"(?<from>{block})\s*:\s*(?<to>{block})";
Console.WriteLine(Regex.IsMatch("12345678 :87654321", pattern));
}
}
The $#"..." is a verbatim interpolated string literal, where escape sequences are treated as combinations of a literal backslash and a char after it. Make sure to define literal { with {{ and } with }} (e.g. $#"(?:{block}){{5}}" to repeat a block 5 times).
For older C# versions, use string.Format:
var pattern = string.Format(#"(?<from>{0})\s*:\s*(?<to>{0})", block);
as is suggested in Mattias's answer.
If I am understanding your question correctly, you want to reuse certain patterns to construct a bigger pattern?
string f = #"fc\d+/";
string e = #"\d+";
Regex regexObj = new Regex(f+e);
Other than this, using backreferences will only help if you are trying to match the exact same string that you have previously matched somewhere in your regex.
e.g.
/\b([a-z])\w+\1\b/
Will only match : text, spaces in the above text :
This is a sample text which is not the title since it does not end with 2 spaces.
There is no such predefined class. I think you can simplify it using ignore-case option, e.g.:
(?i)(?<from>[0-9a-z]{1,8})\s*:\s*(?<to>[0-9a-z]{1,8})
To reuse regex named capture group use this syntax: \k<name> or \k'name'
So the answer is:
(?<from>[0-9a-fA-F]{1,8})\s*:\s*\k<from>
More info: http://www.regular-expressions.info/named.html
usually i can workaround and get everything works by myself, but this one is kinda tricky, even msdn references and examples confuses more than helps.
i have testing some codes and stuck at mixing a capture grouping for changing with a non-capturing group, to stop the matchings when i wish
a simpler code that i want to change is:
stats = "label:100,value:7878,label:110,value:7879,something,label:200,value:8888";
valor = "value:8080";
i know if i use
pattern = #"value:(\d+)";
i can change every value number to 8080 when i do
Regex.Replace(stats, pattern, valor);
but i need he stops changing these when find 'something' string
i managed to change every single char to 'valor' until he finds 'something' using
pattern = #"^(?:(?!something).)*";
is there a way to only change 'value:(\d+)' numbers to 'valor' , along with the ?:(?!something) to stop the matchings in the same sentence?
ive seen lots of examples but they never said something like this so i dunno if its possible to merge both conditions at same time
You can make use of a look-behind solution that makes sure there is no something before the value:
(?<!\bsomething\b.*)value:\d+
See demo
Note that something is matched as a whole word due to \b word boundaries.
The result of replace operation:
Note that (?:(?!something).) is very inefficient and should be used when no other means works. In .NET, there is a powerful variable-width look-behind, which is the right tool for this task.
Also note that if you are not using capture group backreferences, you do not need those capturing groups in your pattern (I remove parentheses from around \d+).
I am trying to validate a textbox that it must contain the values starting from fixed word "temp", User must enter temp before entering any other thing in the textbox.
Please help.
Regards.
Have you tried regular expressions? Regular expressions are a way to see if a string contains a specified sequence of characters, and is much more robust than a simple 'search'! They're a powerful tool and I would suggest google for a tutorial.
I noticed you said this is client side, so here's a page describing regexp in javascript. I haven't used regular expressions in javascript, but they can be very useful. Of course, regular expressions are also available in C#.
Basically you'll want to use "^temp" as your pattern. The '^' will make sure that the matching starts at the beginning of the string you're testing, and check to see if 'temp' is there. If the pattern doesn't match, the string doesn't have 'temp' at the start of it.
var stringToTest = "TemP this should match"
var pattern = /^temp/i
var result = pattern.test(stringToTest)
Above is a simple example that I pulled from W3Schools. As you see, the pattern uses '^temp' as its pattern, and it uses the modifier 'i' to make the check case-insensitive, so that it doesn't matter how the user types in 'temp'(Could be Temp, temP, teMp, teMP, tEmp, etc).
I am new to regular expressions and the one that i have written might be a very simple one but donot know where I am wrong.
#"^([a-zA-Z._]+)#([\d]+)"
This RE is for the following string:
somename#somenumber
Now i am trying to retrieve the somename and somenumber. This is what i did:
ac.name = m.Groups[0].Value;
ac.number = m.Groups[1].Value;
Here ac.name reads the complete string, and ac.number reads somenumber. Where am I wrong in ac.name?
i guess the regex is correct, the problem is, you get the ac.name not from group 1 but group(0), which is the whole string. try this:
ac.name = m.Groups[1].Value;
ac.number = m.Groups[2].Value;
This regex is correct. I think your mistake is in somewhere else. You seem to use C#. So, you should think about the regex usage in the language.
Looking to the code sample in MSDN, you need to use 1-based indexes while accessing Groups instead of zero-based (as also Kent suggested). So, use this:
String name = m.Groups[1].Value;
String number = m.Groups[2].Value;
use this regex (\w+)#(\d+([.,]\d+)?)
Groups[1] will be contain name
Groups[2] will be contain number
I think you should move the + into the capture group:
#"^([a-zA-Z._]+)#([\d]+)"
If this is C#, try without the ^
([a-zA-Z\._]+)#([\d]+)
I just tried it out and it groups properly
Update: escaped the .
If you want only one match (and hence the ^ in original expression), use .Match instead of .Matches method. See MSDN documentation on Regular Expression Classes.