For example, in this string with no \s:
abodnpjdcqe
only d should be matched.
But in my case there are thousands of different characters, is it possible to use ONLY regxp to match all characters that appear in the string more than once? It seems that all other problems use other tools.
It is possible to find characters that are present two times in a string as anubhava demonstrates it, and I don't see any other regex pattern to do it.
However, there are problems with an only regex way:
The complexity of this kind of pattern is very high, and you will experience problems (with backtracking limits and execution time) if your string is long and if there are few duplicates.
This way is unable to see if a duplicate character have been already found. For example the string a123a456a789a, the pattern will return a three times instead of one. If your goal is to obtain a list of unique duplicate characters, it can be problematic (but easy to solve programmatically)
So, to answer your question: my answer is no.
a simple way, to do it with code is to loop over the characters of your string and to build an associative array where the keys are the characters and the values the number of occurences. Then, removes each item that has the value 1 and extract the keys.
Note: you can solve the problem of duplicate results (2.) using this pattern:
(.)(?=(?:(?!\1).)*\1(?:(?!\1).)*$)
or if possessive quantifiers are available:
(.)(?=(?:(?!\1).)*+\1(?:(?!\1).)*+$)
but I'm afraid that the complexity may be even more high.
So, using your favorite language stay from far the best way.
You can use this regex:
([a-zA-Z])(?=.*\1)
Explanation:
Regex uses ([a-zA-Z]) to match any letter and captures it as group #1 i.e. \1
A positive lookahead (?=.*\1) then makes sure this match is successful only when it is followed by at least one of the backreference \1 i.e. the character itself.
RegEx Demo
Related
This question sounds like a duplicate, but I've looked at a LOT of similar questions, and none fit the bill either because they restrict their question to a very specific example, or to a specific usercase (e.g: single chars only) or because you need substitution for a successful approach, or because you'd need to use a programming language (e.g: C#'s split, or Match().Value).
I want to be able to get the reverse of any arbitrary Regex expression, so that everything is matched EXCEPT the found match.
For example, let's say I want to find the reverse of the Regex "over" in "The cow jumps over the moon", it would match The cow jumps and also match the moon.
That's only a simple example of course. The Regex could be something more messy such as "o.*?m", in which case the matches would be: The c, ps, and oon.
Here is one possible solution I found after ages of hunting. Unfortunately, it requires the use of substitution in the replace field which I was hoping to keep clear. Also, everything else is matched, but only a character by character basis instead of big chunks.
Just to stress again, the answer should be general-purpose for any arbitrary Regex, and not specific to any particular example.
From post: I want to be able to get the reverse of any arbitrary Regex expression, so that everything is matched EXCEPT the found match.
The answer -
A match is Not Discontinuous, it is continuous !!
Each match is a continuous, unbroken substring. So, within each match there
is no skipping anything within that substring. Whatever matched the
regular expression is included in a particular match result.
So within a single Match, there is no inverting (i.e. match not this only) that can extend past
a negative thing.
This is a Tennant of Regular Expressions.
Further, in this case, since you only want all things NOT something, you have
to consume that something in the process.
This is easily done by just capturing what you want.
So, even with multiple matches, its not good enough to say (?:(?!\bover\b).)+
because even though it will match up to (but not) over, on the next match
it will match ver ....
There are ways to avoid this that are tedious, requiring variable length lookbehinds.
But, the easiest way is to match up to over, then over, then the rest.
Several constructs can help. One is \K.
Unfortunately, there is no magical recipe to negate a pattern.
As you mentioned it in your question when you have an efficient pattern you use with a match method, to obtain the complementary, the more easy (and efficient) way is to use a split method with the same pattern.
To do it with the pattern itself, workarounds are:
1. consuming the characters that match the pattern
"other content" is the content until the next pattern or the end of the string.
alternation + capture group:
(pattern)|other content
Then you must check if the capture group exists to know which part of the alternation succeeds.
"other content" can be for example described in this way: .*?(?=pattern|$)
With PCRE and Perl, you can use backtracking control verbs to avoid the capture group, but the idea is the same:
pattern(*SKIP)(*FAIL)|other content
With this variant, you don't need to check anything after, since the first branch is forced to fail.
or without alternation:
((?:pattern)*)(other content)
variant in PCRE, Perl, or Ruby with the \K feature:
(?:pattern)*\Kother content
Where \K removes all on the left from the match result.
2. checking characters of the string one by one
(?:(?!pattern).)*
if this way is very simple to write (if the lookahead is available), it has the inconvenient to be slow since each positions of the string are tested with the lookahead.
The amount of lookahead tests can be reduced if you can use the first character of the pattern (lets say "a"):
[^a]*(?:(?!pattern)a[^a]*)*
3. list all that is not the pattern.
using character classes
Lets say your pattern is /hello/:
([^h]|h(([^eh]|$)|e(([^lh]|$)|l(([^lh]|$)|l([^oh]|$))))*
This way becomes quickly fastidious when the number of characters is important, but it can be useful for regex flavors that haven't many features like POSIX regex.
How do I write a regular expression where x is a string whose characters are either a, b, c but no two consecutive characters are the same
For example
abcacb is true
acbaac is false
^(?!.*(.)\1)[abc]+$ works if you follow the original question exactly. However, this does not work/check multiple "words" of characters a/b/c, ie. "abc cba".
The way it works is it asserts that any character is not followed by itself by utilizing a capture group inside a lookahead and that the entire string consists only of characters "a", "b", or "c".
Since the number of chars is limited, you can get away without a back reference in the look ahead:
^(?!.*(aa|bb|cc)[abc]*$
But I like tenub's answer better :)
using negative lookbehind: ^([abc]([abc](?<!(aa|bb|cc)))*)?$ TRY HERE
using negative lookahead: ^(((?!(aa|bb|cc))[abc])*[abc])?$ TRY HERE
Prefer either (both do the same job but differently) if you are going to use this regex as a part of some bigger regex that you might be creating.
In short, this is reusable. Copy & paste and it will do its work without disturbing any regex that is present around it.
In my humble opinion, regexes provided in #tenub and #Bohemian are not reusable which can cause bugs.
Note: empty string ("") will pass these 2 regexes. If you don't want it to, remove ? from regex.
I've just learned about these two concepts in more detail. I've always been good with RegEx, and it seems I've never seen the need for these 2 zero width assertions.
I'm pretty sure I'm wrong, but I do not see why these constructs are needed. Consider this example:
Match a 'q' which is not followed by a 'u'.
2 strings will be the input:
Iraq
quit
With negative lookahead, the regex looks like this:
q(?!u)
Without it, it looks like this:
q[^u]
For the given input, both of these regex give the same results (i.e. matching Iraq but not quit) (tested with perl). The same idea applies to lookbehinds.
Am I missing a crucial feature that makes these assertions more valuable than the classic syntax?
Why your test probably worked (and why it shouldn't)
The reason you were able to match Iraq in your test might be that your string contained a \n at the end (for instance, if you read it from the shell). If you have a string that ends in q, then q[^u] cannot match it as the others said, because [^u] matches a non-u character - but the point is there has to be a character.
What do we actually need lookarounds for?
Obviously in the above case, lookaheads are not vital. You could workaround this by using q(?:[^u]|$). So we match only if q is followed by a non-u character or the end of the string. There are much more sophisticated uses for lookaheads though, which become a pain if you do them without lookaheads.
This answer tries to give an overview of some important standard situations which are best solved with lookarounds.
Let's start with looking at quoted strings. The usual way to match them is with something like "[^"]*" (not with ".*?"). After the opening ", we simply repeat as many non-quote characters as possible and then match the closing quote. Again, a negated character class is perfectly fine. But there are cases, where a negated character class doesn't cut it:
Multi-character delimiters
Now what if we don't have double-quotes to delimit our substring of interest, but a multi-character delimiter. For instance, we are looking for ---sometext---, where single and double - are allowed within sometext. Now you can't just use [^-]*, because that would forbid single -. The standard technique is to use a negative lookahead at every position, and only consume the next character, if it is not the beginning of ---. Like so:
---(?:(?!---).)*---
This might look a bit complicated if you haven't seen it before, but it's certainly nicer (and usually more efficient) than the alternatives.
Different delimiters
You get a similar case, where your delimiter is only one character but could be one of two (or more) different characters. For instance, say in our initial example, we want to allow for both single- and double-quoted strings. Of course, you could use '[^']*'|"[^"]*", but it would be nice to treat both cases without an alternative. The surrounding quotes can easily be taken care of with a backreference: (['"])[^'"]*\1. This makes sure that the match ends with the same character it began with. But now we're too restrictive - we'd like to allow " in single-quoted and ' in double-quoted strings. Something like [^\1] doesn't work, because a backreference will in general contain more than one character. So we use the same technique as above:
(['"])(?:(?!\1).)*\1
That is after the opening quote, before consuming each character we make sure that it is not the same as the opening character. We do that as long as possible, and then match the opening character again.
Overlapping matches
This is a (completely different) problem that can usually not be solved at all without lookarounds. If you search for a match globally (or want to regex-replace something globally), you may have noticed that matches can never overlap. I.e. if you search for ... in abcdefghi you get abc, def, ghi and not bcd, cde and so on. This can be problem if you want to make sure that your match is preceded (or surrounded) by something else.
Say you have a CSV file like
aaa,111,bbb,222,333,ccc
and you want to extract only fields that are entirely numerical. For simplicity, I'll assume that there is no leading or trailing whitespace anywhere. Without lookarounds, we might go with capturing and try:
(?:^|,)(\d+)(?:,|$)
So we make sure that we have the start of a field (start of string or ,), then only digits, and then the end of a field (, or end of string). Between that we capture the digits into group 1. Unfortunately, this will not give us 333 in the above example, because the , that precedes it was already part of the match ,222, - and matches cannot overlap. Lookarounds solve the problem:
(?<=^|,)\d+(?=,|$)
Or if you prefer double negation over alternation, this is equivalent to
(?<![^,])\d+(?![^,])
In addition to being able to get all matches, we get rid of the capturing which can generally improve performance. (Thanks to Adrian Pronk for this example.)
Multiple independent conditions
Another very classic example of when to use lookarounds (in particular lookaheads) is when we want to check multiple conditions on an input at the same time. Say we want to write a single regex that makes sure our input contains a digit, a lower case letter, an upper case letter, a character that is none of those, and no whitespace (say, for password security). Without lookarounds you'd have to consider all permutations of digit, lower case/upper case letter, and symbol. Like:
\S*\d\S*[a-z]\S*[A-Z]\S*[^0-9a-zA_Z]\S*|\S*\d\S*[A-Z]\S*[a-z]\S*[^0-9a-zA_Z]\S*|...
Those are only two of the 24 necessary permutations. If you also want to ensure a minimum string length in the same regex, you'd have to distribute those in all possible combinations of the \S* - it simply becomes impossible to do in a single regex.
Lookahead to the rescue! We can simply use several lookaheads at the beginning of the string to check all of these conditions:
^(?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[^0-9a-zA-Z])(?!.*\s)
Because the lookaheads don't actually consume anything, after checking each condition the engine resets to the beginning of the string and can start looking at the next one. If we wanted to add a minimum string length (say 8), we could simply append (?=.{8}). Much simpler, much more readable, much more maintainable.
Important note: This is not the best general approach to check these conditions in any real setting. If you are making the check programmatically, it's usually better to have one regex for each condition, and check them separately - this let's you return a much more useful error message. However, the above is sometimes necessary, if you have some fixed framework that lets you do validation only by supplying a single regex. In addition, it's worth knowing the general technique, if you ever have independent criteria for a string to match.
I hope these examples give you a better idea of why people would like to use lookarounds. There are a lot more applications (another classic is inserting commas into numbers), but it's important that you realise that there is a difference between (?!u) and [^u] and that there are cases where negated character classes are not powerful enough at all.
q[^u] will not match "Iraq" because it will look for another symbol.
q(?!u) however, will match "Iraq":
regex = /q[^u]/
/q[^u]/
regex.test("Iraq")
false
regex.test("Iraqf")
true
regex = /q(?!u)/
/q(?!u)/
regex.test("Iraq")
true
Well, another thing along with what others mentioned with the negative lookahead, you can match consecutive characters (e.g. you can negate ui while with [^...], you cannot negate ui but either u or i and if you try [^ui]{2}, you will also negate uu, ii and iu.
The whole point is to not "consume" the next character(s), so that it can be e.g. captured by another expression that comes afterwards.
If they're the last expression in the regex, then what you've shown are equivalent.
But e.g. q(?!u)([a-z]) would let the non-u character be part of the next group.
I know it is quite some weird goal here but for a quick and dirty fix for one of our system we do need to not filter any input and let the corruption go into the system.
My current regex for this is "\^.*"
The problem with that is that it does not match characters as planned ... but for one match it does work. The string that make it not work is ^#jj (basically anything that has ^ ... ).
What would be the best way to not match any characters now ? I was thinking of removing the \ but only doing this will transform the "not" into a "start with" ...
The ^ character doesn't mean "not" except inside a character class ([]). If you want to not match anything, you could use a negative lookahead that matches anything: (?!.*).
A simple and cheap regex that will never match anything is to match against something that is simply unmatchable, for example: \b\B.
It's simply impossible for this regex to match, since it's a contradiction.
References
regular-expressions.info\Word Boundaries
\B is the negated version of \b. \B matches at every position where \b does not.
Another very well supported and fast pattern that would fail to match anything that is guaranteed to be constant time:
$unmatchable pattern $anything goes here etc.
$ of course indicates the end-of-line. No characters could possibly go after $ so no further state transitions could possibly be made. The additional advantage are that your pattern is intuitive, self-descriptive and readable as well!
tldr; The most portable and efficient regex to never match anything is $- (end of line followed by a char)
Impossible regex
The most reliable solution is to create an impossible regex. There are many impossible regexes but not all are as good.
First you want to avoid "lookahead" solutions because some regex engines don't support it.
Then you want to make sure your "impossible regex" is efficient and won't take too much computation steps to match... nothing.
I found that $- has a constant computation time ( O(1) ) and only takes two steps to compute regardless of the size of your text (https://regex101.com/r/yjcs1Z/3).
For comparison:
$^ and $. both take 36 steps to compute -> O(1)
\b\B takes 1507 steps on my sample and increase with the number of character in your string -> O(n)
Empty regex (alternative solution)
If your regex engine accepts it, the best and simplest regex to never match anything might be: an empty regex .
Instead of trying to not match any characters, why not just match all characters? ^.*$ should do the trick. If you have to not match any characters then try ^\j$ (Assuming of course, that your regular expression engine will not throw an error when you provide it an invalid character class. If it does, try ^()$. A quick test with RegexBuddy suggests that this might work.
^ is only not when it's in class (such as [^a-z] meaning anything but a-z). You've turned it into a literal ^ with the backslash.
What you're trying to do is [^]*, but that's not legal. You could try something like
" {10000}"
which would match exactly 10,000 spaces, if that's longer than your maximum input, it should never be matched.
((?iLmsux))
Try this, it matches only if the string is empty.
Interesting ... the most obvious and simple variant:
~^
.
https://regex101.com/r/KhTM1i/1
requiring usually only one computation step (failing directly at the start and being computational expensive only if the matched string begins with a long series of ~) is not mentioned among all the other answers ... for 12 years.
You want to match nothing at all? Neg lookarounds seems obvious, but can be slow, perhaps ^$ (matches empty string only) as an alternative?
I need to validate VARCHAR field.
conditions are: field must contain atleast 2 AlphaNumeric characters
so please any one give the Regular Expression for the above conditions
i wrote below Expression but it will check atleast 2 letters are alphanumeric. and if my input will have other than alphanumeric it is nt validating.
'^[a-zA-Z0-9]{2,}$'
please help.........
[a-zA-Z0-9].*[a-zA-Z0-9]
The easy way: At least two alnum's anywhere in the string.
Answer to comments
I never did (nor intended to do) any benchmarking. Therefore - and given that we know nothing of OP's environment - I am not one to judge whether a non-greedy version ([a-zA-Z0-9].*?[a-zA-Z0-9]) will be more efficient. I do believe, however, that the performance impact is totally negligible :)
I would probably use this regular expression:
[a-zA-Z0-9][^a-zA-Z0-9]*[a-zA-Z0-9]
How broad is your definition of alphanumeric? For US ASCII, see the answers above. For a more cosmopolitan view, use one of
[[:alnum:]].*[[:alnum:]]
or
[^\W_].*[^\W_]
The latter works because \w matches a "word character," alphanumerics and underscore. Use a double-negative to exclude the underscore: "not not-a-word-character and not underscore."
As simple as
'\w.*\w'
In response to a comment, here's a performance comparison for the greedy [a-zA-Z0-9].*[a-zA-Z0-9] and the non-greedy [a-zA-Z0-9].*?[a-zA-Z0-9].
The greedy version will find the first alphanumeric, match all the way to the end, and backtrack to the last alphanumeric, finding the longest possible match. For a long string, it is the slowest version. The non greedy version finds the first alphanumeric, and tries not to match the following symbols until another alphanumeric is found (that is, for every letter it matches the empty string, tries to match [a-zA-Z0-9], fails, and matches .).
Benchmarking (empirical results):
In case the alphanumeric are very far away, the greedy version is faster (even faster than Gumbo's version).
In case the alphanumerics are close to each other, the greedy version is significantly slower.
The test: http://jsbin.com/eletu/4
Compares 3 versions:
[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]
Conclusion: none. As always, you should check against typical data.