regex to highlight sentences longer than n words

Question

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.

Answer 1

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.