Related
https://regex101.com/r/sB9wW6/1
(?:(?<=\s)|^)#(\S+) <-- the problem in positive lookbehind
Working like this on prod: (?:\s|^)#(\S+), but I need a correct start index (without space).
Here is in JS:
var regex = new RegExp(/(?:(?<=\s)|^)#(\S+)/g);
Error parsing regular expression: Invalid regular expression:
/(?:(?<=\s)|^)#(\S+)/
What am I doing wrong?
UPDATE
Ok, no lookbehind in JS :(
But anyways, I need a regex to get the proper start and end index of my match. Without leading space.
Make sure you always select the right regex engine at regex101.com. See an issue that occurred due to using a JS-only compatible regex with [^] construct in Python.
JS regex - at the time of answering this question - did not support lookbehinds. Now, it becomes more and more adopted after its introduction in ECMAScript 2018. You do not really need it here since you can use capturing groups:
var re = /(?:\s|^)#(\S+)/g;
var str = 's #vln1\n#vln2\n';
var res = [];
while ((m = re.exec(str)) !== null) {
res.push(m[1]);
}
console.log(res);
The (?:\s|^)#(\S+) matches a whitespace or the start of string with (?:\s|^), then matches #, and then matches and captures into Group 1 one or more non-whitespace chars with (\S+).
To get the start/end indices, use
var re = /(\s|^)#\S+/g;
var str = 's #vln1\n#vln2\n';
var pos = [];
while ((m = re.exec(str)) !== null) {
pos.push([m.index+m[1].length, m.index+m[0].length]);
}
console.log(pos);
BONUS
My regex works at regex101.com, but not in...
First of all, have you checked the Code Generator link in the Tools pane on the left?
All languages - "Literal string" vs. "String literal" alert - Make sure you test against the same text used in code, literal string, at the regex tester. A common scenario is copy/pasting a string literal value directly into the test string field, with all string escape sequences like \n (line feed char), \r (carriage return), \t (tab char). See Regex_search c++, for example. Mind that they must be replaced with their literal counterparts. So, if you have in Python text = "Text\n\n abc", you must use Text, two line breaks, abc in the regex tester text field. Text.*?abc will never match it although you might think it "works". Yes, . does not always match line break chars, see How do I match any character across multiple lines in a regular expression?
All languages - Backslash alert - Make sure you correctly use a backslash in your string literal, in most languages, in regular string literals, use double backslash, i.e. \d used at regex101.com must written as \\d. In raw string literals, use a single backslash, same as at regex101. Escaping word boundary is very important, since, in many languages (C#, Python, Java, JavaScript, Ruby, etc.), "\b" is used to define a BACKSPACE char, i.e. it is a valid string escape sequence. PHP does not support \b string escape sequence, so "/\b/" = '/\b/' there.
All languages - Default flags - Global and Multiline - Note that by default m and g flags are enabled at regex101.com. So, if you use ^ and $, they will match at the start and end of lines correspondingly. If you need the same behavior in your code check how multiline mode is implemented and either use a specific flag, or - if supported - use an inline (?m) embedded (inline) modifier. The g flag enables multiple occurrence matching, it is often implemented using specific functions/methods. Check your language reference to find the appropriate one.
line-breaks - Line endings at regex101.com are LF only, you can't test strings with CRLF endings, see regex101.com VS myserver - different results. Solutions can be different for each regex library: either use \R (PCRE, Java, Ruby) or some kind of \v (Boost, PCRE), \r?\n, (?:\r\n?|\n)/(?>\r\n?|\n) (good for .NET) or [\r\n]+ in other libraries (see answers for C#, PHP). Another issue related to the fact that you test your regex against a multiline string (not a list of standalone strings/lines) is that your patterns may consume the end of line, \n, char with negated character classes, see an issue like that. \D matched the end of line char, and in order to avoid it, [^\d\n] could be used, or other alternatives.
php - You are dealing with Unicode strings, or want shorthand character classes to match Unicode characters, too (e.g. \w+ to match Стрибижев or Stribiżew, or \s+ to match hard spaces), then you need to use u modifier, see preg_match() returns 0 although regex testers work - To match all occurrences, use preg_match_all, not preg_match with /...pattern.../g, see PHP preg_match to find multiple occurrences and "Unknown modifier 'g' in..." when using preg_match in PHP?- Your regex with inline backreference like \1 refuses to work? Are you using a double quoted string literal? Use a single-quoted one, see Backreference does not work in PHP
phplaravel - Mind you need the regex delimiters around the pattern, see https://stackoverflow.com/questions/22430529
python - Note that re.search, re.match, re.fullmatch, re.findall and re.finditer accept the regex as the first argument, and the input string as the second argument. Not re.findall("test 200 300", r"\d+"), but re.findall(r"\d+", "test 200 300"). If you test at regex101.com, please check the "Code Generator" page. - You used re.match that only searches for a match at the start of the string, use re.search: Regex works fine on Pythex, but not in Python - If the regex contains capturing group(s), re.findall returns a list of captures/capture tuples. Either use non-capturing groups, or re.finditer, or remove redundant capturing groups, see re.findall behaves weird - If you used ^ in the pattern to denote start of a line, not start of the whole string, or used $ to denote the end of a line and not a string, pass re.M or re.MULTILINE flag to re method, see Using ^ to match beginning of line in Python regex
- If you try to match some text across multiple lines, and use re.DOTALL or re.S, or [\s\S]* / [\s\S]*?, and still nothing works, check if you read the file line by line, say, with for line in file:. You must pass the whole file contents as the input to the regex method, see Getting Everything Between Two Characters Across New Lines. - Having trouble adding flags to regex and trying something like pattern = r"/abc/gi"? See How to add modifers to regex in python?
c#, .net - .NET regex does not support possessive quantifiers like ++, *+, ??, {1,10}?, see .NET regex matching digits between optional text with possessive quantifer is not working - When you match against a multiline string and use RegexOptions.Multiline option (or inline (?m) modifier) with an $ anchor in the pattern to match entire lines, and get no match in code, you need to add \r? before $, see .Net regex matching $ with the end of the string and not of line, even with multiline enabled - To get multiple matches, use Regex.Matches, not Regex.Match, see RegEx Match multiple times in string - Similar case as above: splitting a string into paragraphs, by a double line break sequence - C# / Regex Pattern works in online testing, but not at runtime - You should remove regex delimiters, i.e. #"/\d+/" must actually look like #"\d+", see Simple and tested online regex containing regex delimiters does not work in C# code - If you unnecessarily used Regex.Escape to escape all characters in a regular expression (like Regex.Escape(#"\d+\.\d+")) you need to remove Regex.Escape, see Regular Expression working in regex tester, but not in c#
dartflutter - Use raw string literal, RegExp(r"\d"), or double backslashes (RegExp("\\d")) - https://stackoverflow.com/questions/59085824
javascript - Double escape backslashes in a RegExp("\\d"): Why do regex constructors need to be double escaped?
- (Negative) lookbehinds unsupported by most browsers: Regex works on browser but not in Node.js - Strings are immutable, assign the .replace result to a var - The .replace() method does change the string in place - Retrieve all matches with str.match(/pat/g) - Regex101 and Js regex search showing different results or, with RegExp#exec, RegEx to extract all matches from string using RegExp.exec- Replace all pattern matches in string: Why does javascript replace only first instance when using replace?
javascriptangular - Double the backslashes if you define a regex with a string literal, or just use a regex literal notation, see https://stackoverflow.com/questions/56097782
java - Word boundary not working? Make sure you use double backslashes, "\\b", see Regex \b word boundary not works - Getting invalid escape sequence exception? Same thing, double backslashes - Java doesn't work with regex \s, says: invalid escape sequence - No match found is bugging you? Run Matcher.find() / Matcher.matches() - Why does my regex work on RegexPlanet and regex101 but not in my code? - .matches() requires a full string match, use .find(): Java Regex pattern that matches in any online tester but doesn't in Eclipse - Access groups using matcher.group(x): Regex not working in Java while working otherwise - Inside a character class, both [ and ] must be escaped - Using square brackets inside character class in Java regex - You should not run matcher.matches() and matcher.find() consecutively, use only if (matcher.matches()) {...} to check if the pattern matches the whole string and then act accordingly, or use if (matcher.find()) to check if there is a single match or while (matcher.find()) to find multiple matches (or Matcher#results()). See Why does my regex work on RegexPlanet and regex101 but not in my code?
scala - Your regex attempts to match several lines, but you read the file line by line (e.g. use for (line <- fSource.getLines))? Read it into a single variable (see matching new line in Scala regex, when reading from file)
kotlin - You have Regex("/^\\d+$/")? Remove the outer slashes, they are regex delimiter chars that are not part of a pattern. See Find one or more word in string using Regex in Kotlin - You expect a partial string match, but .matchEntire requires a full string match? Use .find, see Regex doesn't match in Kotlin
mongodb - Do not enclose /.../ with single/double quotation marks, see mongodb regex doesn't work
c++ - regex_match requires a full string match, use regex_search to find a partial match - Regex not working as expected with C++ regex_match - regex_search finds the first match only. Use sregex_token_iterator or sregex_iterator to get all matches: see What does std::match_results::size return? - When you read a user-defined string using std::string input; std::cin >> input;, note that cin will only get to the first whitespace, to read the whole line properly, use std::getline(std::cin, input); - C++ Regex to match '+' quantifier - "\d" does not work, you need to use "\\d" or R"(\d)" (a raw string literal) - This regex doesn't work in c++ - Make sure the regex is tested against a literal text, not a string literal, see Regex_search c++
go - Double backslashes or use a raw string literal: Regular expression doesn't work in Go - Go regex does not support lookarounds, select the right option (Go) at regex101.com before testing! Regex expression negated set not working golang
groovy - Return all matches: Regex that works on regex101 does not work in Groovy
r - Double escape backslashes in the string literal: "'\w' is an unrecognized escape" in grep - Use perl=TRUE to PCRE engine ((g)sub/(g)regexpr): Why is this regex using lookbehinds invalid in R?
oracle - Greediness of all quantifiers is set by the first quantifier in the regex, see Regex101 vs Oracle Regex (then, you need to make all the quantifiers as greedy as the first one)] - \b does not work? Oracle regex does not support word boundaries at all, use workarounds as shown in Regex matching works on regex tester but not in oracle
firebase - Double escape backslashes, make sure ^ only appears at the start of the pattern and $ is located only at the end (if any), and note you cannot use more than 9 inline backreferences: Firebase Rules Regex Birthday
firebasegoogle-cloud-firestore - In Firestore security rules, the regular expression needs to be passed as a string, which also means it shouldn't be wrapped in / symbols, i.e. use allow create: if docId.matches("^\\d+$").... See https://stackoverflow.com/questions/63243300
google-data-studio - /pattern/g in REGEXP_REPLACE must contain no / regex delimiters and flags (like g) - see How to use Regex to replace square brackets from date field in Google Data Studio?
google-sheets - If you think REGEXEXTRACT does not return full matches, truncates the results, you should check if you have redundant capturing groups in your regex and remove them, or convert the capturing groups to non-capturing by add ?: after the opening (, see Extract url domain root in Google Sheet
sed - Why does my regular expression work in X but not in Y?
word-boundarypcrephp - [[:<:]] and [[:>:]] do not work in the regex tester, although they are valid constructs in PCRE, see https://stackoverflow.com/questions/48670105
snowflake-cloud-data-platform snowflake-sql - If you are writing a stored procedure, and \\d does not work, you need to double them again and use \\\\d, see REGEX conversion of VARCHAR value to DATE in Snowflake stored procedure using RLIKE not consistent.
I have the simple regex #"[a-zA-Z]" to match all characters a-z in a string but I also need math operators (*, /, +, -). I was reading over the documentation on msdn but I got lost relatively fast due to the math operators being used as other tokens in the regex
This solution works:
#"[A-Za-z\*\+\-\/]"
Thanks for the help and resources everyone.
The correct answer is
#"[A-Za-z*+/-]"
Or #"[A-Za-z-*+/]", or #"[-A-Za-z*+/]", or #"[A-Za-z*\-+/]".
Or, shorten it with a case-insensitive modifier: #"(?i)[A-Z*+/-]" (or use a corresponding RegexOptions.IgnoreCase with #"[A-Z*+/-]" since it seems you are using C#).
Inside a character class, the unescaped hyphen should either be at the start or final position to be treated as a literal, or right after a range or shorthand class. Otherwise, it must be escaped. Also, ] must be escaped if not at the beginning of the character class. Other characters do not have to be escpaed inside a character class.
To test, use an appropriate online regex tester. You need one for .NET, see Regex demo at RegexStorm.
I am trying to get the search position of keywords in google using below regex:
string lookup = "(<h3 class=\"r\"><a href=\")(\\w+[a-zA-Z0-9.-?=/]*)";
But this is not working for urls having hypens(-) like:
www.example-xyz.com
Can anyone help me to fix this?
Escape your hyphen with backslash and escape that escaping backslash with another backslash:
string lookup = "(<h3 class=\"r\"><a href=\")(\\w+[a-zA-Z0-9.\\-?=/]*)";
Since - means a range within a [], you need to escape it with a backslash.
string lookup = "(<h3 class=\"r\"><a href=\")(\\w+[a-zA-Z0-9.\-?=/]*)";
By the way, there are many questions on stackoverflow about matching urls with regex, search tags [regex] and [url] to have a look if you want a more refined regex.
Read a decent book on Regular Expressions, like Jeffrey E.F. Friedl's "Mastering Regular Expressions".
Not only it will show you that the - makes a character range in a character class -
[a-z]
and so must be escaped -
[a\-z]
or put at the beginning -
[-az]
or at the end -
[az-]
when meant verbatim, but also that it is usually a mistake to parse such markup (a context-free language, in Chomsky terms) with one Regular Expression alone.
You are looking for a markup parser (like BeautifulSoup or lxml, but in C#), and RFC 3986, Appendix B for a proper URI-matching expression instead.
can some tell how can i write regular expression matching abc.com.ae or abc.net.af or anything ,ae which is the last in the string is optional
this is successfull with / but not . don't know why
^[a-z]{1,25}.[a-z]{3}$
answer
^[a-z]{1,30}\.[a-z]{3}((\.)[a-z]{2})?$
The answer is: write \. instead of ., because . means “any character”.
You can match specific characters using the \u escape code followed immediately by the unicode number for that character in hex format and it must also be four digits only. I think a full stop is \u002E.
In your example where '/' works but not '.', this is because a '/' is recognised as a normal character and does not need to be escaped whereas the full stop has a different meaning in c# regex (it matches any character).
If youre still not sure, here is a useful guide to regex in C#: http://www.radsoftware.com.au/articles/regexlearnsyntax.aspx
And this one has examples of regex for URLs: http://www.radsoftware.com.au/articles/regexsyntaxadvanced.aspx
I sounds like you need some environment to learn regular expressions.
The best way to learn regular expressions is with an interactive regular expression simulator: type in an expression, and it will give you the results, instantly.
There are a few free (and not so free) regular expression simulators available:
Free web version from http://lumadis.be/regex/test_regex.php.
RegExBuddy from http://www.regexbuddy.com/. I am not affiliated with this program, but I have used it with good success in the past.
Greetings.
I've been tasked with debugging part of an application that involves a Regex -- but, I have never dealt with Regex before. Two questions:
1) I know that the regexes are supposed to be testing whether or not two strings are equivalent, but what specifically do the two regex statements, below, mean in plain English?
2) Does anyone have a recommendation on websites / sources where I can learn more about Regexes? (preferably in C#)
if (Regex.IsMatch(testString, #"^(\s*?)(" + tag + #")(\s*?),", RegexOptions.IgnoreCase))
{
result = true;
}
else if (Regex.IsMatch(testString, #",(\s*?)(" + tag + #")(\s*?),", RegexOptions.IgnoreCase))
{
result = true;
}
It's going to be difficult to tell what that regex means, without knowing what's in tag. In fact, it looks like that regex is broken (or, at least, doesn't properly escape inputs).
Roughly speaking, for the first regex:
The ^ says to match at the beginning of the string.
The (...) sets up a capturing group (which is available, although this example apparently doesn't use it).
The \s matches any white space characters (spaces, tabs, etc.)
The *? matches zero or more of the previous character (in this case, whitespace), and because it has a question-mark, it matches the minimum number of characters needed to make the rest of the expression work.
The (" + tag + #") inserts the contents of the tag into the regex. As I mention, that's dangerous, without escaping.
The (\s*?) matches the same as the before (the minimum number of whitespace characters)
The , matches a trailing comma.
The second regex is very similar, but looks for a starting comma (rather than the beginning of the string).
I like the Python documentation for Regular Expressions, but it looks like this site
has a pretty good, basic introduction, with C# examples.
One word - Cribsheet (or is that two?) :)
I'm not c# savvy but I can recommend an awesome guide to regular expressions that I use for Bash and Java programming. It applies to pretty much all languages:
http://www.amazon.com/Mastering-Regular-Expressions-Jeffrey-Friedl/dp/0596528124/ref=tmm_pap_title_0
It is totally worth $30 to own this book. It is VERY thorough and helped my fundamental understanding of Regex a lot.
-Ryan
Since you specifically tagged C#, I recommend the Regex Hero as a tool you can use to play around with them since it's running on .NET. It also lets you toggle the different RegexOptions flags as you would pass them into the constructor when creating a new Regex.
Also, if you're using a version of Visual Studio 2010 that supports extensions, I would take a look at the Regex Editor extension... it will popup whenever you type new Regex( and offer you some guidance and autocomplete for your regex pattern.
Using The Regex Coach
The regular expression is a sequence consisting of the expression '(\s*?)', the expression '(tag)', the expression '(\s*?)', and the character ','.
where (\s*?) is defined as The regular expression is a repetition which matches a whitespace character as often as necessary.
the second one matches a , at the start too
As for good learning websites, I like www.regular-expressions.info/
Super simple version:
At the start of a string 0 or more spaces, whatever Tag is, 0 or More spaces, a comma.
the second one is
a comma, 0 or more spaces, whatever Tag is, 0 or More spaces, a comma.
Once you have the very basic idea about regex (it's full of resources over there) I recommend you to use Expresso for creating your regular expressions.
Expresso editor is equally suitable as a teaching tool for the beginning user of regular expressions or as a full-featured development environment for the experienced programmer or web designer with an extensive knowledge of regular expressions.
Your premise is not correct. Regular expressions are not used to tell if two strings are equivalent, but rather if the input string matches a certain pattern.
The first test above looks for any text that does not contain "zero or more whitespace charaters" searching "non-greedy". Then matches the text of the variable "tag" in the middle, then "zero or more whitespace characters, non greedy" again.
The second one is very similar, except that it allows for beginning whitespace as long as it follows a comma.
It is hard to explain "non-greedy" in this context, especially involving whitespace characters, so look here for more information.
A regular expression is a way to describe a set of strings that have some particular characteristics.
They don't merely need just to compare two strings.. what you usually do it to test if a string matches a particular regular expression. They can also be used to do simple parsing of a string in tokens that respect some patterns..
The good thing about regexps is that they allow you to express certain constraints inside a string keeping it general and able to match a group of strings that respect those constraints.. then they follow a formal specification that doesn't leave ambiguities around..
Here you can find a comparison table of various regular expression languages in many different programming languages and a specific guide for C# if you follow its link.
Usually the implementations for the various languages are quite similar since the syntax is somewhat standardized from the theoretical topics regexps come from, so any tutorial about regexp will be fine, then you'll just need to get into C# API.
1) The first regex is trying to do a case-insensitive match starting at the beginning of the test string. It then matches optional whitespace, followed by whatever is in tag, followed by optional whitespace then finally a comma.
The second matches a string containing a comma, followed by optional whitespace, followed by whatever is in tag, followed by optional whitespace then finally a comma.
Thought it's for C# I recommend picking up the Perl Pocket Reference which has a great Regex syntax reference. It helped my out a lot when I was learning regexes 14 years ago.
http://www.myregextester.com/ is a decent regular expression tester that also has an explain option for C# regexps - For Instance check out this example:
The regular expression:
(?-imsx:^(\s*?)(tagtext)(\s*?),)
matches as follows:
NODE EXPLANATION
----------------------------------------------------------------------
(?-imsx: group, but do not capture (case-sensitive)
(with ^ and $ matching normally) (with . not
matching \n) (matching whitespace and #
normally):
----------------------------------------------------------------------
^ the beginning of the string
----------------------------------------------------------------------
( group and capture to \1:
----------------------------------------------------------------------
\s*? whitespace (\n, \r, \t, \f, and " ") (0
or more times (matching the least amount
possible))
----------------------------------------------------------------------
) end of \1
----------------------------------------------------------------------
( group and capture to \2:
----------------------------------------------------------------------
tagtext 'tagtext'
----------------------------------------------------------------------
) end of \2
----------------------------------------------------------------------
( group and capture to \3:
----------------------------------------------------------------------
\s*? whitespace (\n, \r, \t, \f, and " ") (0
or more times (matching the least amount
possible))
----------------------------------------------------------------------
) end of \3
----------------------------------------------------------------------
, ','
----------------------------------------------------------------------
) end of grouping
----------------------------------------------------------------------
A regular expression does not tell you if two strings match, but rather if a given string matches a pattern.
This site is my favorite for learning and testing regular expressions:
http://gskinner.com/RegExr/
It allows you to interactively test regular expressions as you write them, and provides a built-in tutorial.
Although it doesn't use C#, Rejex is a simple tool for testing and learning about regular expressions which includes a quick reference for the special characters
It looks like that they are trying to match some kind of list of words delimited by colons (UPDATE: commas).
The first one is probably matching first item and the second one some item after the first one excluding the last one. I hope you will understand :).
A good source of information about regular expressions is at http://www.regular-expressions.info/
also a great site to test your regular expressions with extra info: http://regex101.com/