I am trying to write a regex expression that can be used to identify long sentences in a document. I my case a scientific manuscript. I aim to be doing that either in libre office or any text editor with regex search.
So far I got the following expression to work on most occasions:
(\[*\(*[\w|\-|–|−|\/|≥|≤|’|“|”|μ]+\%*\)*\]*,*\:*\s+){24,}?(\[*\(*[\w|\-|–|−|\/|≥|≤|’|“|”|μ]+\%*\)*\]*[\.|?|!|$])
btw, I got inspired from this post
It contains:
group1:
(\[*\(*[\w|\-|–|−|\/|≥|≤|’|“|”|μ]+\%*\)*\]*,*\:*\s+)
a repetition element (stating how many words n - 1):
{24,}?
group2:
(\[*\(*[\w|\-|–|−|\/|≥|≤|’|“|”|μ]+\%*\)*\]*[\.|?|!|$])
The basic functioning is:
group1 matches any number of word characters OR other characters that are present in the text followed by one or more spaces
group1 has to be repeated 24 times (or as many as you want the sentences to be long)
group2 matches any number of word characters OR other characters that are present in the text followed by a full stop, exclamation mark, question mark or paragraph break.
Any string that fulfills all the above would then be highlighted.
What I can't solve so far is to make it work when a dot appears in the text with another meaning than a full stop. Things like: i.e., e.g., et al., Fig., 1.89, etc....
Also I don't like that I had to manually adjust it to be able to handle sentences that contain non-word characters such as , [ ( % - # µ " ' and so on. I would have to extend the expression every time I come across some other uncommon character.
I'd be happy for any help or suggestions of other ways to solve this.
You can do a lot with the swiss-army-knife that is regular expressions, but the problem you've presented approaches regex's limits. Some of the things you want to detect can probably be handled with really small changes, while others are a bit harder. If your goal is to have some kind of tool that accurately measures sentence length for every possible mutation of characters, you'll probably need to move outside LibreOffice to a dedicated custom piece of software or a third-party tool.
But, that said, there are a few tricks you can worm into your existing regex to make it work better, if you want to avoid programming or another tool. Let's look at a few of the techniques that might be useful to you:
You can probably tweak your regex for a few special cases, like Fig. and Mr., by including them directly. Where you currently have [\w|\-|–|−|\/|≥|≤|’|“|”|μ]+, which is basically [\w]+ with a bunch of other "special" characters, you could use something like ([\w|...]+|Mr\.|Mrs\.|Miss\.|Fig\.) (substituting in all the special characters where I wrote ..., of course). Regexes are "greedy" algorithms, and will try to consume as much of the text as possible, so by including special "dot words" directly, you can make the regex "skip over" certain period characters that are problematic in your text. Make sure that when you want to add a "period to skip" that you always precede it with a backslash, like in i\.e\., so that it doesn't get treated as the special "any" character.
A similar trick can capture numbers better by assuming that digits followed by a period followed by more digits are supposed to "eat" the period: ([\w|...]+|\d+\.\d+|...) That doesn't handle everything, and if your document authors are writing stuff like 0. in the middle of sentences then you have a tough problem, but it can at least handle pi and e properly.
Also, right now, your regex consumes characters until it reaches any terminating punctuation character — a ., or !, or ?, or the end of the document. That's a problem for things like i.e., and 3.14, since as far as your regex is concerned, the sentence stops at the .. You could require your regex to only stop the sentence once ._ is reached — a period followed by a space. That wouldn't fix mismatches for words like Mr., but it would treat "words" like 3.14 as a word instead of as the end of a sentence, which is closer than you currently are. To do this, you'll have to include an odd sequence as part of the "word" regex, something like (\.[^ ]), which says "dot followed by not-a-space" is part of the word; and then you'll have to change the terminating sequence to (\. |!|?|$). Repeat the changes similarly for ! and ?.
Another useful trick is to take advantage of character-code ranges, instead of encoding each special character directly. Right now, you're doing it the hard way, by spelling out every accented character and digraph and diacritic in the universe. Instead, you could just say that everything that's a "special character" is considered to be part of the "word": Instead of [\w|\-|–|−|\/|≥|≤|’|“|”|μ]+, write [\w|\-|\/|\u0080-\uFFFF], which captures every character except emoji and a few from really obscure dead languages. LibreOffice seems to have Unicode support, so using \uXXXX patterns should work inside [ character ranges ].
This is probably enough to make your regex somewhat acceptable in LibreOffice, and might even be enough to answer your question. But if you're really intent on doing more complex document analysis like this, you may be better off exporting the document as plain text and then running a specialized tool on it.
I need some help with building up my regex.
What I am trying to do is match a specific part of text with unpredictable parts in between the fixed words. An example is the sentence one gets when replying to an email:
On date at time person name has written:
The cursive parts are variable, might contains spaces or a new line might start from this point.
To get this, I built up my regex as such: On[\s\S]+?at[\s\S]+?person[\s\S]+?has written:
Basically, the [\s\S]+? is supposed to fill in any letter, number, space or break/new line as I am unable to predict what could be between the fixed words tha I am sure will always be there.
Now comes the hard part, when I would add the word "On" somewhere in the text above the sentence that I want to match, the regex now matches a much bigger text than I want. This is due to the use of [\s\S]+.
How am I able to make my regex match as less characters as possible? Using "?" before the "+" to make it lazy does not help.
Example is here with words "From - This - Point - Everything:". Cases are ignored.
Correct: https://regexr.com/3jdek.
Wrong because of added "From": https://regexr.com/3jdfc
The regex is to be used in VB.NET
A more real life, with html tags, can be found here. Here, I avoided using [\s\S]+? or (.+)?(\r)?(\n)?(.+?)
Correct: https://regexr.com/3jdd1
Wrong: https://regexr.com/3jdfu after adding certain parts of the regex in the text above. Although, in html, barely possible to occur as the user would never write the matching tag himself, I do want to make sure my regex is correctjust in case
These things are certain: I know with what the part of text starts, no matter where in respect to the entire text, I know with what the part of text ends, and there are specific fixed words that might make the regex more reliable, but they can be ommitted. Any text below the searched part is also allowed to be matched, but no text above may be matched at all
Another example where it goes wrong: https://regexr.com/3jdli. Basically, I have less to go with in this text, so the regex has less tokens to work with. Adding just the first < already makes the regex take too much.
From my own experience, most problems are avoided when making sure I do not use any [\s\S]+? before I did a (\r)?(\n)? first
[\s\S] matches all character because of union of two complementary sets, it is like . with special option /s (dot matches newlines). and regex are greedy by default so the largest match will be returned.
Following correct link, the token just after the shortest match must be geschreven, so another way to write without using lazy expansion, which is more flexible is to prepend the repeated chracter set by a negative lookahead inside loop,
so
<blockquote type="cite" [^>]+?>[^O]+?Op[^h]+?heeft(.+?(?=geschreven))geschreven:
becomes
<blockquote type="cite" [^>]+?>[^O]+?Op[^h]+?heeft((?:(?!geschreven).)+)geschreven:
(?: ) is for non capturing the group which just encapsulates the negative lookahead and the . (which can be replaced by [\s\S])
(?! ) inside is the negative lookahead which ensures current position before next character is not the beginning of end token.
Following comments it can be explicitly mentioned what should not appear in repeating sequence :
From(?:(?!this)[\s\S])+this(?:(?!point)[\s\S])+point(?:(?!everything)[\s\S])+everything:
or
From(?:(?!From|this)[\s\S])+this(?:(?!point)[\s\S])+point(?:(?!everything)[\s\S])+everything:
or
From(?:(?!From|this)[\s\S])+this(?:(?!this|point)[\s\S])+point(?:(?!everything)[\s\S])+everything:
to understand what the technic (?:(?!tokens)[\s\S])+ does.
in the first this can't appear between From and this
in the second From or this can't appear between From and this
in the third this or point can't appear between this and point
etc.
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$|\s+(?=\s)
Which makes sure to leave one space behind so that you can replace with nothing and get "this is a sentence".
Splitting
Well, somewhere else where positive lookarounds might be useful is where, say you have a string ABC12DE3456FGHI789 and want to get the letters+digits apart, that is you want to get ABC12, DE3456 and FGHI789. You can easily do use the regex:
(?<=[0-9])(?=[A-Z])
While if you use ([A-Z]+[0-9]+) (i.e. the captured groups are put back in the resulting list/array/etc, you will be getting empty elements as well.
Note that this could be done with a match as well, with [A-Z]+[0-9]+
If I had to mention negative lookarounds, this post would have been even longer :)
Keep in mind that a positive/negative lookaround is the same for a regex engine. The goal of lookarounds is to perform a check somewhere in your "regular expression".
One of the main interest is to capture something without using capturing parenthesis (capturing the whole pattern), example:
string: aaabbbccc
regex: (?<=aaa)bbb(?=ccc)
(you obtain the result with the whole pattern)
instead of: aaa(bbb)ccc
(you obtain the result with the capturing group.)
So I wanted to limit a textbox which contains an apartment number which is optional.
Here is the regex in question:
([0-9]{1,4}[A-Z]?)|([A-Z])|(^$)
Simple enough eh?
I'm using these tools to test my regex:
Regex Analyzer
Regex Validator
Here are the expected results:
Valid
"1234A"
"Z"
"(Empty string)"
Invalid
"A1234"
"fhfdsahds527523832dvhsfdg"
Obviously if I'm here, the invalid ones are accepted by the regex. The goal of this regex is accept either 1 to 4 numbers with an optional letter, or a single letter or an empty string.
I just can't seem to figure out what's not working, I mean it is a simple enough regex we have here. I'm probably missing something as I'm not very good with regexes, but this syntax seems ok to my eyes. Hopefully someone here can point to my error.
Thanks for all help, it is greatly appreciated.
You need to use the ^ and $ anchors for your first two options as well. Also you can include the second option into the first one (which immediately matches the third variant as well):
^[0-9]{0,4}[A-Z]?$
Without the anchors your regular expression matches because it will just pick a single letter from anywhere within your string.
Depending on the language, you can also use a negative look ahead.
^[0-9]{0,4}[A-Za-z](?!.*[0-9])
Breakdown:
^[0-9]{0,4} = This look for any number 0 through 4 times at the beginning of the string
[A-Za-z] = This look for any characters (Both cases)
(?!.*[0-9]) = This will only allow the letters if there are no numbers anywhere after the letter.
I haven't quite figured out how to validate against a null character, but that might be easier done using tools from whatever language you are using. Something along this logic:
if String Doesn't equal $null Then check the Rexex
Something along those lines, just adjusted for however you would do it in your language.
I used RegEx Skinner to validate the answers.
Edit: Fixed error from comments