I'm hoping we have some regular expression guru's here that might be able to help me - a regex newbie - solve a problem.
I know some people will want to know some background info on this issue:
Regex Flavor: Basic Regex, being used in a Vertica Database using the REGEXP_REPLACE function.
The regex I am using is working great with one exception.
I have a rule that I'm trying to implement, related to stripping the numbers from text, where any number that is part of a word, e.g. table5, go2market, 33monroe, room222, etc. is ignored and NOT filtered.
Here is what I started with for detecting numbers:
[-+]?[0-9]*\.?[0-9]
That seems to work pretty well, including handling directly adjacent commas and parentheses for example.
But all cases where there is a number that is part of alphabetic text is also being detected, which fails the rule that it cannot be a part of a word, and by word, I mean any alphabetic text.
So, in searching for solutions, I happened upon this regex that seems to work well detecting those specific cases where numbers appear next to, or in, any string of characters:
((?:[a-zA-Z]+[0-9]|[0-9]+[a-zA-Z])[a-zA-Z0-9]*)
My thought was that maybe I could add this as an INVERTED match to my original regex, to allow it to still select standalone numbers while ignoring those that were a part of a word, like so:
[-+]?[0-9]^((?:[a-zA-Z]+[0-9]|[0-9]+[a-zA-Z])[a-zA-Z0-9]*)*\.?[0-9]^((?:[a-zA-Z]+[0-9]|[0-9]+[a-zA-Z])[a-zA-Z0-9]*)
Unfortunately however, it breaks the original detection of standalone numbers.
:(
I'm hoping there is someone here that can spot what I'm doing wrong, and help me identify the right solution?
Thanks in advance!
According to Vertica documentation, the regex flavour seems to follow the Perl syntax. In this case you can use negative lookarounds and in particular a negative lookbehind: (?<!\w) (not preceded with a word character.)
Lookarounds are only tests and don't consume characters.
You can also use a negative lookahead to test the right part, (?!\w) (not followed by a word character), but it's more simple to use a word boundary since the pattern ends with a digit (that is also a word character):
(?<!\w)[-+]?\d*\.?\d+\b
In the worst case, if you have something like v1.0 in your string and you want to avoid it, you can try to use the bactracking control verbs (*SKIP) and (*FAIL). (*FAIL) forces the pattern to fail and (*SKIP) skips all the already matched positions before it. I hope vertica supports these Perl regex features.
Something like:
\p{L}+[-+]?\d*\.?\d+(*SKIP)(*FAIL)|[-+]?\d*\.?\d+(*SKIP)(?!\p{L})
Many regex questions lately have some kind of look-around element in the query that appears to me is not necessary to the success of the match. Is there some teaching resource that is promoting them? I am trying to figure out what kinds of cases you would be better off using a positive look ahead/behind. The main application I can see is when trying to not match an element. But, for example, this query from a recent question has a simple solution to capturing the .*, but why would you use a look behind?
(?<=<td><a href="\/xxx\.html\?n=[0-9]{0, 5}">).*(?=<\/a><span
And this one from another question:
$url = "www.example.com/id/1234";
preg_match("/\d+(?<=id\/[\d])/",$url,$matches);
When is it truly better to use a positive look-around? Can you give some examples?
I realize this is bordering on an opinion-based question, but I think the answers would be really instructive. Regex is confusing enough without making things more complicated... I have read this page and am more interested in some simple guidelines for when to use them rather than how they work.
Thanks for all the replies. In addition to those below, I recommend checking out m.buettner's great answer here.
You can capture overlapping matches, and you can find matches which could lie in the lookarounds of other matches.
You can express complex logical assertions about your match (because many engines let you use multiple lookbehind/lookahead assertions which all must match in order for the match to succeed).
Lookaround is a natural way to express the common constraint "matches X, if it is followed by/preceded by Y". It is (arguably) less natural to add extra "matching" parts that have to be thrown out by postprocessing.
Negative lookaround assertions, of course, are even more useful. Combined with #2, they can allow you do some pretty wizard tricks, which may even be hard to express in usual program logic.
Examples, by popular request:
Overlapping matches: suppose you want to find all candidate genes in a given genetic sequence. Genes generally start with ATG, and end with TAG, TAA or TGA. But, candidates could overlap: false starts may exist. So, you can use a regex like this:
ATG(?=((?:...)*(?:TAG|TAA|TGA)))
This simple regex looks for the ATG start-codon, followed by some number of codons, followed by a stop codon. It pulls out everything that looks like a gene (sans start codon), and properly outputs genes even if they overlap.
Zero-width matching: suppose you want to find every tr with a specific class in a computer-generated HTML page. You might do something like this:
<tr class="TableRow">.*?</tr>(?=<tr class="TableRow">|</table>)
This deals with the case in which a bare </tr> appears inside the row. (Of course, in general, an HTML parser is a better choice, but sometimes you just need something quick and dirty).
Multiple constraints: suppose you have a file with data like id:tag1,tag2,tag3,tag4, with tags in any order, and you want to find all rows with tags "green" and "egg". This can be done easily with two lookaheads:
(.*):(?=.*\bgreen\b)(?=.*\begg\b)
There are two great things about lookaround expressions:
They are zero-width assertions. They require to be matched, but they consume nothing of the input string. This allows to describe parts of the string which will not be contained in a match result. By using capturing groups in lookaround expressions, they are the only way to capture parts of the input multiple times.
They simplify a lot of things. While they do not extend regular languages, they easily allow to combine (intersect) multiple expressions to match the same part of a string.
Well one simple case where they are handy is when you are anchoring the pattern to the start or finish of a line, and just want to make sure that something is either right ahead or behind the pattern you are matching.
I try to address your points:
some kind of look-around element in the query that appears to me is not necessary to the success of the match
Of course they are necessary for the match. As soon as a lookaround assertions fails, there is no match. They can be used to ensure conditions around the pattern, that have additionally to be true. The whole regex does only match, if:
The pattern does fit and
The lookaround assertions are true.
==> But the returned match is only the pattern.
When is it truly better to use a positive look-around?
Simple answer: when you want stuff to be there, but you don't want to match it!
As Bergi mentioned in his answer, they are zero width assertions, this means they don't match a character sequence, they just ensure it is there. So the characters inside a lookaround expression are not "consumed", the regex engine continues after the last "consumed" character.
Regarding your first example:
(?<=<td><a href="\/xxx\.html\?n=[0-9]{0, 5}">).*(?=<\/a><span
I think there is a misunderstanding on your side, when you write "has a simple solution to capturing the .*". The .* is not "captured", it is the only thing that the expression does match. But only those characters are matched that have a "<td><a href="\/xxx\.html\?n=[0-9]{0, 5}">" before and a "<\/a><span" after (those two are not part of the match!).
"Captured" is only something that has been matched by a capturing group.
The second example
\d+(?<=id\/[\d])
Is interesting. It is matching a sequence of digits (\d+) and after the sequence, the lookbehind assertion checks if there is one digit with "id/" before it. Means it will fail if there is more than one digit or if the text "id/" before the digit is missing. Means this regex is matching only one digit, when there is fitting text before.
teaching resources
www.regular-expressions.info
perlretut on Looking ahead and looking behind
I'm assuming you understand the good uses of lookarounds, and ask why they are used with no apparent reason.
I think there are four main categories of how people use regular expressions:
Validation
Validation is usually done on the whole text. Lookarounds like you describe are not possible.
Match
Extracting a part of the text. Lookarounds are used mainly due to developer laziness: avoiding captures.
For example, if we have in a settings file with the line Index=5, we can match /^Index=(\d+)/ and take the first group, or match /(?<=^Index=)\d+/ and take everything.
As other answers said, sometimes you need overlapping between matches, but these are relatively rare.
Replace
This is similar to match with one difference: the whole match is removed and is being replaced with a new string (and some captured groups).
Example: we want to highlight the name in "Hi, my name is Bob!".
We can replace /(name is )(\w+)/ with $1<b>$2</b>,
but it is neater to replace /(?<=name is )\w+/ with <b>$&</b> - and no captures at all.
Split
split takes the text and breaks it to an array of tokens, with your pattern being the delimiter. This is done by:
Find a match. Everything before this match is token.
The content of the match is discarded, but:
In most flavors, each captured group in the match is also a token (notably not in Java).
When there are no more matches, the rest of the text is the last token.
Here, lookarounds are crucial. Matching a character means removing it from the result, or at least separating it from its token.
Example: We have a comma separated list of quoted string: "Hello","Hi, I'm Jim."
Splitting by comma /,/ is wrong: {"Hello", "Hi, I'm Jim."}
We can't add the quote mark, /",/: {"Hello, "Hi, I'm Jim."}
The only good option is lookbehind, /(?<="),/: {"Hello", "Hi, I'm Jim."}
Personally, I prefer to match the tokens rather than split by the delimiter, whenever that is possible.
Conclusion
To answer the main question - these lookarounds are used because:
Sometimes you can't match text that need.
Developers are shiftless.
Lookaround assertions can also be used to reduce backtracking which can be the main cause for a bad performance in regexes.
For example: The regex ^[0-9A-Z]([-.\w]*[0-9A-Z])*#(1) can also be written ^[0-9A-Z][-.\w]*(?<=[0-9A-Z])#(2) using a positive look behind (simple validation of the user name in an e-mail address).
Regex (1) can cause a lot of backtracking essentially because [0-9A-Z] is a subset of [-.\w] and the nested quantifiers. Regex (2) reduces the excessive backtracking, more information here Backtracking, section Controlling Backtracking > Lookbehind Assertions.
For more information about backtracking
Best Practices for Regular Expressions in the .NET Framework
Optimizing Regular Expression Performance, Part II: Taking Charge of Backtracking
Runaway Regular Expressions: Catastrophic Backtracking
I typed this a while back but got busy (still am, so I might take a while to reply back) and didn't get around to post it. If you're still open to answers...
Is there some teaching resource that is promoting them?
I don't think so, it's just a coincidence I believe.
But, for example, this query from a recent question has a simple solution to capturing the .*, but why would you use a look behind?
(?<=<td><a href="\/xxx\.html\?n=[0-9]{0, 5}">).*(?=<\/a><span
This is most probably a C# regex, since variable width lookbehinds are not supported my many regex engines. Well, the lookarounds could be certainly avoided here, because for this, I believe it's really simpler to have capture groups (and make the .* lazy as we're at it):
(<td><a href="\/xxx\.html\?n=[0-9]{0,5}">).*?(<\/a><span)
If it's for a replace, or
<td><a href="\/xxx\.html\?n=[0-9]{0,5}">(.*?)<\/a><span
for a match. Though an html parser would definitely be more advisable here.
Lookarounds in this case I believe are slower. See regex101 demo where the match is 64 steps for capture groups but 94+19 = 1-3 steps for the lookarounds.
When is it truly better to use a positive look-around? Can you give some examples?
Well, lookarounds have the property of being zero-width assertions, which mean they don't really comtribute to matches while they contribute onto deciding what to match and also allows overlapping matches.
Thinking a bit about it, I think, too, that negative lookarounds get used much more often, but that doesn't make positive lookarounds less useful!
Some 'exploits' I can find browsing some old answers of mine (links below will be demos from regex101) follow. When/If you see something you're not familiar about, I probably won't be explaining it here, since the question's focused on positive lookarounds, but you can always look at the demo links I provided where there's a description of the regex, and if you still want some explanation, let me know and I'll try to explain as much as I can.
To get matches between certain characters:
In some matches, positive lookahead make things easier, where a lookahead could do as well, or when it's not so practical to use no lookarounds:
Dog sighed. "I'm no super dog, nor special dog," said Dog, "I'm an ordinary dog, now leave me alone!" Dog pushed him away and made his way to the other dog.
We want to get all the dog (regardless of case) outside quotes. With a positive lookahead, we can do this:
\bdog\b(?=(?:[^"]*"[^"]*")*[^"]*$)
to ensure that there are even number of quotes ahead. With a negative lookahead, it would look like this:
\bdog\b(?!(?:[^"]*"[^"]*")*[^"]*"[^"]*$)
to ensure that there are no odd number of quotes ahead. Or use something like this if you don't want a lookahead, but you'll have to extract the group 1 matches:
(?:"[^"]+"[^"]+?)?(\bdog\b)
Okay, now say we want the opposite; find 'dog' inside the quotes. The regex with the lookarounds just need to have the sign inversed, first and second:
\bdog\b(?!(?:[^"]*"[^"]*")*[^"]*$)
\bdog\b(?=(?:[^"]*"[^"]*")*[^"]*"[^"]*$)
But without the lookaheads, it's not possible. the closest you can get is maybe this:
"[^"]*(\bdog\b)[^"]*"
But this doesn't get all the matches, or you can maybe use this:
"[^"]*?(\bdog\b)[^"]*?(?:(\bdog\b)[^"]*?)?"
But it's just not practical for more occurrences of dog and you get the results in variables with increasing numbers... And this is indeed easier with lookarounds, because they are zero width assertions, you don't have to worry about the expression inside the lookaround to match dog or not, or the regex wouldn't have obtained all the occurrences of dog in the quotes.
Of course now, this logic can be extended to groups of characters, such as getting specific patterns between words such as start and end.
Overlapping matches
If you have a string like:
abcdefghijkl
And want to extract all the consecutive 3 characters possible inside, you can use this:
(?=(...))
If you have something like:
1A Line1 Detail1 Detail2 Detail3 2A Line2 Detail 3A Line3 Detail Detail
And want to extract these, knowing that each line starts with #A Line# (where # is a number):
1A Line1 Detail1 Detail2 Detail3
2A Line2 Detail
3A Line3 Detail Detail
You might try this, which fails because of greediness...
[0-9]+A Line[0-9]+(?: \w+)+
Or this, which when made lazy no more works...
[0-9]+A Line[0-9]+(?: \w+)+?
But with a positive lookahead, you get this:
[0-9]+A Line[0-9]+(?: \w+)+?(?= [0-9]+A Line[0-9]+|$)
And appropriately extracts what's needed.
Another possible situation is one where you have something like this:
#ff00fffirstword#445533secondword##008877thi#rdword#
Which you want to convert to three pairs of variables (first of the pair being a # and some hex values (6) and whatever characters after them):
#ff00ff and firstword
#445533 and secondword#
#008877 and thi#rdword#
If there were no hashes inside the 'words', it would have been enough to use (#[0-9a-f]{6})([^#]+), but unfortunately, that's not the case and you have to resort to .*? instead of [^#]+, which doesn't quite yet solve the issue of stray hashes. Positive lookaheads however make this possible:
(#[0-9a-f]{6})(.+?)(?=#[0-9a-f]{6}|$)
Validation & Formatting
Not recommended, but you can use positive lookaheads for quick validations. The following regex for instance allow the entry of a string containing at least 1 digit and 1 lowercase letter.
^(?=[^0-9]*[0-9])(?=[^a-z]*[a-z])
This can be useful when you're checking for character length but have patterns of varying length in the a string, for example, a 4 character long string with valid formats where # indicates a digit and the hyphen/dash/minus - must be in the middle:
##-#
#-##
A regex like this does the trick:
^(?=.{4}$)\d+-\d+
Where otherwise, you'd do ^(?:[0-9]{2}-[0-9]|[0-9]-[0-9]{2})$ and imagine now that the max length was 15; the number of alterations you'd need.
If you want a quick and dirty way to rearrange some dates in the 'messed up' format mmm-yyyy and yyyy-mm to a more uniform format mmm-yyyy, you can use this:
(?=.*(\b\w{3}\b))(?=.*(\b\d{4}\b)).*
Input:
Oct-2013
2013-Oct
Output:
Oct-2013
Oct-2013
An alternative might be to use a regex (normal match) and process separately all the non-conforming formats separately.
Something else I came across on SO was the indian currency format, which was ##,##,###.### (3 digits to the left of the decimal and all other digits groupped in pair). If you have an input of 122123123456.764244, you expect 1,22,12,31,23,456.764244 and if you want to use a regex, this one does this:
\G\d{1,2}\K\B(?=(?:\d{2})*\d{3}(?!\d))
(The (?:\G|^) in the link is only used because \G matches only at the start of the string and after a match) and I don't think this could work without the positive lookahead, since it looks forward without moving the point of replacement.)
Trimming
Suppose you have:
this is a sentence
And want to trim all the spaces with a single regex. You might be tempted to do a general replace on spaces:
\s+
But this yields thisisasentence. Well, maybe replace with a single space? It now yields " this is a sentence " (double quotes used because backticks eats spaces). Something you can however do is this:
^\s*|\s$|\s+(?=\s)
Which makes sure to leave one space behind so that you can replace with nothing and get "this is a sentence".
Splitting
Well, somewhere else where positive lookarounds might be useful is where, say you have a string ABC12DE3456FGHI789 and want to get the letters+digits apart, that is you want to get ABC12, DE3456 and FGHI789. You can easily do use the regex:
(?<=[0-9])(?=[A-Z])
While if you use ([A-Z]+[0-9]+) (i.e. the captured groups are put back in the resulting list/array/etc, you will be getting empty elements as well.
Note that this could be done with a match as well, with [A-Z]+[0-9]+
If I had to mention negative lookarounds, this post would have been even longer :)
Keep in mind that a positive/negative lookaround is the same for a regex engine. The goal of lookarounds is to perform a check somewhere in your "regular expression".
One of the main interest is to capture something without using capturing parenthesis (capturing the whole pattern), example:
string: aaabbbccc
regex: (?<=aaa)bbb(?=ccc)
(you obtain the result with the whole pattern)
instead of: aaa(bbb)ccc
(you obtain the result with the capturing group.)
First timer and relatively inexperienced with RegEx and Notepad++. What I am trying to do is replace everything but the policy numbers in these two firewall session. Mind you, I have a list multiple lists 700+ lines long so I want to replace everything in one pass, leaving just the policy number for each line.
id 1978781/s23,vsys 0,flag 00200440/4000/0003,policy 4332,time 5972, dip 0 module 0
id 1997645/s23,vsys 0,flag 00200440/4000/0003,policy 30562,time 6283, dip 0 module 0
There are thousands of different policy numbers, so a simple search wont do.
I would like my lines to look like this after a replace.
4332
30562
After two hours of trying to learn RegEx for this one problem, I realized this its more involved than I expected, and I need to spend time learning this since its a very powerful tool. This could really save a lot of time, which unfortunately I don't have at the moment. I'm looking forward to learning more about RegEx and appreciate any help or direction you could give me.
Given the fact the lines always look the same you can use the following
^.+policy (\d+).+$
Replace by : $1
The dot is a wild card so , .+ means find everything before the word "policy ". Then find a group of digits (\d+ is for finding digits) and save them (thats what the parenthesis are for in many regex engines). Then find all the characters till the end of the line.
The ^ character means start of line. The $ means end of line.
You can try the following:
Find:
^.*policy ([0-9]+).*$
Replace with:
\1
Why does this work?
The dot matches any character, and the star means "zero or more of" the character preceding it. This means that .* matches everything.
What you want is to match everything before and after the policy and erase it, and keep just the policy number, so between your everything matchers you look for the string "policy xxxxx" where the xxxxx are numbers.
Each term surrounded by parenthesis in your regex is saved to be used in the replacement. I put parenthesis around the number matcher, [0-9]+ and then use what was matched in the repace part with \1. If your regex contains several parenthesized parts, you can get them with \1, \2, \3...
Regexes are really powerful, you should read a tutorial about them to learn what they can offer.
I am using a regex to find:
test:?
Followed by any character until it hits the next:
test:?
Now when I run this regex I made:
((?:test:\?)(.*)(?!test:\?))
On this text:
test:?foo2=bar2&baz2=foo2test:?foo=bar&baz=footest:?foo2=bar2&baz2=foo2
I expected to get:
test:?foo2=bar2&baz2=foo2
test:?foo=bar&baz=foo
test:?foo2=bar2&baz2=foo2
But instead it matches everything. Does anyone with more regex experience know where I have gone wrong? I've used regexes for pattern matching before but this is my first experience of lookarounds/aheads.
Thanks in advance for any help/tips/pointers :-)
I guess you could explore a greedy version.
(expanded)
(test:\? (?: (?!test:\?)[\s\S])* )
The Perl program below
#! /usr/bin/env perl
use strict;
use warnings;
$_ = "test:?foo2=bar2&baz2=foo2test:?foo=bar&baz=footest:?foo2=bar2&baz2=foo2";
while (/(test:\? .*?) (?= test:\? | $)/gx) {
print "[$1]\n";
}
produces the desired output from your question, plus brackets for emphasis.
[test:?foo2=bar2&baz2=foo2]
[test:?foo=bar&baz=foo]
[test:?foo2=bar2&baz2=foo2]
Remember that regex quantifiers are greedy and want to gobble up as much as they can without breaking the match. Each subsegment to terminate as soon as possible, which means .*? semantics.
Each subsegment terminates with either another test:? or end-of-string, which we look for with (?=...) zero-width lookahead wrapped around | for alternatives.
The pattern in the code above uses Perl’s /x regex switch for readability. Depending on the language and libraries you’re using, you may need to remove the extra whitespace.
Three issues:
(?!) is a negative lookahead assertion. You want (?=) instead, requiring that what comes next is test:?.
The .* is greedy; you want it non-greedy so that you grab just the first chunk.
You're wanting the last chunk also, so you want to match $ as well at the end.
End result:
(?:test:\?)(.*?)(?=test:\?|$)
I've also removed the outer group, seeing no point in it. All RE engines that I know of let you access group 0 as the full match, or some other such way (though perhaps not when finding all matches). You can put it back if you need to.
(This works in PCRE; not sure if it would work with POSIX regular expressions, as I'm not in the habit of working with them.)
If you're just wanting to split on test:?, though, regular expressions are the wrong tool. Split the strings using your language's inbuilt support for such things.
Python:
>>> re.findall('(?:test:\?)(.*?)(?=test:\?|$)',
... 'test:?foo2=bar2&baz2=foo2test:?foo=bar&baz=footest:?foo2=bar2&baz2=foo2')
['foo2=bar2&baz2=foo2', 'foo=bar&baz=foo', 'foo2=bar2&baz2=foo2']
You probably want ((?:test:\?)(.*?)(?=test:\?)), although you haven't told us what language you're using to drive the regexes.
The .*? matches as few characters as possible without preventing the whole string from matching, where .* matches as many as possible (is greedy).
Depending, again, on what language you're using to do this, you'll probably need to match, then chop the string, then match again, or call some language-specific match_all type function.
By the way, you don't need to anchor a regex using a lookahead (you can just match the pattern to search for, instead), so this will (most likely) do in your case:
test:[?](.*?)test:[?]
Well, here I am back at regex and my poor understanding of it. Spent more time learning it and this is what I came up with:
/(.*)
I basically want the number in this string:
510973
My regex is almost good? my original was:
"/<a href=\"travis.php?theTaco(.*)\">(.*)<\/a>/";
But sometimes it returned me huge strings. So, I just want to get numbers only.
I searched through other posts but there is such a large amount of unrelated material, please give an example, resource, or a link directing to a very related question.
Thank you.
Try using a HTML parser provided by the language you are using.
Reason why your first regex fails:
[0-9999999] is not what you think. It is same as [0-9] which matches one digit. To match a number you need [0-9]+. Also .* is greedy and will try to match as much as it can. You can use .*? to make it non-greedy. Since you are trying to match a number again, use [0-9]+ again instead of .*. Also if the two number you are capturing will be the same, you can just match the first and use a back reference \1 for 2nd one.
And there are a few regex meta-characters which you need to escape like ., ?.
Try:
<a href=\"travis\.php\?theTaco=([0-9]+)\">\1<\/a>
To capture a number, you don't use a range like [0-99999], you capture by digit. Something like [0-9]+ is more like what you want for that section. Also, escaping is important like codaddict said.
Others have already mentioned some issues regarding your regex, so I won't bother repeating them.
There are also issues regarding how you specified what it is you want. You can simply match via
/theTaco=(\d+)/
and take the first capturing group. You have not given us enough information to know whether this suits your needs.