I am trying to create a regex which attempts to match a sentence.
Here is a snippet.
local utf8 = require 'lua-utf8'
function matchsent(text)
local text = text
for sent in utf8.gmatch(text, "[^\r\n]+\.[\r\n ]") do
print(sent)
print('-----')
end
end
However, it does not work like in python for example. I know that Lua uses different set of regex patterns and it's regex capabilities are limited but why does the regex above give me a syntax error? And how a sentence matching regex in Lua would look like?
Note that Lua uses Lua patterns, that are not "regular" expressions as they cannot match a regular language. They can hardly be used to split a text into sentences since you'd need to account for various abbreviations, spacing, case etc. To split a text into sentences, you need an NLP package rather than one or two regexps due to the complexity of the task.
Regarding
why does the regex above give me a syntax error?
you need to escape special symbols with a % symbol in Lua patterns. See an example code:
function matchsent(text)
for sent in string.gmatch(text, '[^\r\n]+%.[\r\n ]') do
print(sent)
print("---")
end
end
matchsent("Some text here.\nShow me")
An online demo
Related
What exactly is parsing? I mean, generally. How different is parsing different from searching? On command line, if I use the grep tool/command; is that parsing?
For example, if I have just one string:
"Hello world! How are you doing today?"
and I tried to search (using grep or any other tool) whether the word "you" is within that string; is that parsing?
What if I do a web search; for example in Google? Is that parsing?
Or is parsing the name of the process that is a part of the process known as "Search"?
The verb "parse" is essentially related to the word "part", as in "part of speech". (See, for example, the on-line etymology dictionary.)
To "parse" a sentence has traditionally meant to break the sentence down into its component parts and identify their relationship with each other. For example, given "I asked a question.", we can parse it into a subject ("I"), a transitive verb in past tense ("asked"), and an object phrase consisting of an article ("a") and a noun ("question"). The parse indicates that the subject performed some action on the object; this is not the same statement as *"A question asked I", and not just because the latter is ungrammatical.
With the advent of computer languages and computational theory, the term "parsing" has been generalized to include analysis of strings which are not human languages. Some people would even use it to simply mean "to divide a string into its component parts", such as "parsing" a line in a CSV file into fields.
It's quite a stretch to apply that to merely searching for a string inside another string, although there may be contexts in which that is an acceptable use of the word. Personally, I would only use it for the action of completely deconstructing a structured string.
Neither of the two main lexer generators commonly referenced, cl-lex and lispbuilder-lexer allow for state variables in the "action blocks", making it impossible to recognize a c-style multi-line comment, for example.
What is a lexer generator in Common Lisp that can recognize a c-style multi-line comment as a token?
Correction: This lexer actually needs to recognize nested, balanced multiline comments (not exactly C-style). So I can't do away with state-variables.
You can recognize a C-style multiline comment with the following regular expression:
[/][*][^*]*[*]+([^*/][^*]*[*]+)*[/]
It should work with any library which uses Posix-compatible extended regex syntax; although a bit hard to read because * is extensively used both as an operator and as a literal character, it uses no non-regular features. It does rely on inverted character classes ([^*], for example) matching the newline character, but afaik that is pretty well universal, even for regex engines in which a wildcard does not match newline.
I am in need of matching Unicode letters, similarly to PCRE's \p{L}.
Now, since Dart's RegExp class is based on ECMAScript's, it doesn't have the concept of \p{L}, sadly.
I'm looking into perhaps constructing a big character class that matches all Unicode letters, but I'm not sure where to start.
So, I want to match letters like:
foobar
מכון ראות
But the R symbol shouldn't be matched:
BlackBerry®
Neither should any ASCII control characters or punctuation marks, etc. Essentially every letter in every language Unicode supports, whether it's å, ä, φ or ת, they should match if they are actual letters.
I know this is an old question. But RegExp now supports unicode categories (since Dart 2.4) so you can do something like this:
RegExp alpha = RegExp(r'\p{Letter}', unicode: true);
print(alpha.hasMatch("f")); // true
print(alpha.hasMatch("ת")); // true
print(alpha.hasMatch("®")); // false
I don't think that complete information about classification of Unicode characters as letters or non-letters is anywhere in the Dart libraries. You might be able to put something together that would mostly work using things in the Intl library, particularly Bidi. I'm thinking that, for example,
isLetter(oneCharacterString) => Bidi.endsWithLtr(oneLetterString) || Bidi.endsWithRTL(oneLetterString);
might do a plausible job. At least it seems to have a number of ranges for valid characters in there. Or you could put together your own RegExp based on the information in _LTR_CHARS and _RTL_CHARS. It explicitly says it's not 100% accurate, but good for most practical purposes.
Looks like you're going to have to iterate through the runes in the string and then check the integer value against a table of unicode ranges.
Golang has some code to generate these tables directly from the unicode source. See maketables.go, and some of the other files in the golang unicode package.
Or take the lazy option, and file a Dart bug, and wait for the Dart team to implement it ;)
There's no support for this yet in Dart or JS.
The Xregexp JS library has support for generating fairly large character class regexps to support something like this. You may be able to generate the regexp, print it and cut and paste it into your app.
I am making something like formula validator and I am using ParseKit framework to accomplish that. My approach is to create proper grammar and when didMatchFormula callback method is called on sample string I assume formula has been found and therefore it is valid.
There is one difficulty however - formula is detected from sample string even if it contains also other characters following formula part. I would need something like greedy mode for matching - an entire string would be matched against formula grammar so that didMatchFormula would be called only if string contains formula and no other characters.
Can you give me some hints how to accomplish that with PaseKit or in other way.
I cannot use regular expressions since my formulas would use recursion and regexp is not a good tool for handling that.
Developer of ParseKit here.
Probably the simplest and most elegant way to do this with ParseKit (or any parsing toolkit) is to design your formula language have a terminator char after every statement. This would be the same concept as ; terminating statements in most C-like programming languages.
Here's an example toy formula language which uses . as the statement terminator:
#start = lang;
lang = statment+;
statment = Word+ terminator;
terminator = '.';
Notice how I have designed the language so that your "greedy" requirement is an inherent feature of the language. Think about it – if the input string ends with any junk content which is not a valid statement ending in a ., my lang production will not find a match and the parse will fail.
With this type of design, you won't need any "greedy" features in the parsking toolkit you use. Rather, your requirement will be naturally met by your language design.
I've been playing with this for an hour or tow and have found myself at a road block with the Lua pattern matching utilities. I am attempting to match all quoted text in a string and replace it if needed.
The pattern I have come up with so far is: (\?[\"\'])(.-)%1
This works in some cases but, not all cases:
Working: "This \"is a\" string of \"text to\" test with"
Not Working: "T\\\"his \"is\' a\" string\" of\' text\" to \"test\" wit\\\"h"
In the not working example I would like it to match to (I made a function that gets the matches I desire, I'm just looking for a pattern to use with gsub and curious if a lua pattern can do this):
string
a" string" of
is' a" string" of' text
test
his "is' a" string" of' text" to "test" wit
I'm going to continue to use my function instead for the time being, but am curious if there is a pattern I could/should be using and i'm just missing something with patterns.
(a few edits b/c I forgot about stackoverflows formating)
(another edit to make a non-html example since it was leading to assumptions that I was attempting to parse html)
Trying to match escaped, quoted text using regular expressions is like trying to remove the daisies (and only the daises) from a field using a lawnmower.
I made a function that gets the matches I desire
This is the correct move.
I'm curious if a lua pattern can do this
From a practical point of view, even if a pattern can do this, you don't want to. From a theoretical point of view, you are trying to find a double quote that is preceded by an even number of backslashes. This is definitely a regular language, and the regular expression you want would be something like the following (Lua quoting conventions)
[[[^\](\\)*"(.-[^\](\\)*)"]]
And the quoted string would be result #2. But Lua patterns are not full regular expressions; in particular, you cannot put a * after a parenthesized pattern.
So my guess is that this problem cannot be solved using Lua patterns, but since Lua patterns are not a standard thing in automata theory, I'm not aware of any body of proof technique that you could use to prove it.
The issue with escaped quotes is that, in general, if there's an odd number of backslashes before the quote, then it's escaped, and if there's an even number, it's not. I do not believe that Lua pattern-matching is powerful enough to represent this condition, so if you need to parse text like this, then you should seek another way. Perhaps you can iterate through the string and parse it, or you could find each quote in turn and read backwards, counting the backslashes until you find a non-backslash character (or the beginning of the string).
If you absolutely must use patterns for some reason, you could try doing this in a multi-step process. First, gsub for all occurrences of two backslashes in a row, and replace them with some sentinel value. This must be a value that does not already occur in the string. You could try something like "\001" if you know this string doesn't contain non-printable characters. Anyway, once you've replaced all sequences of two backslashes in a row, any backslashes left are escaping the following character. Now you can apply your original pattern, and then finally you can replace all instances of your sentinel value with two backslashes again.
Lua's pattern language is adequate for many simple cases. And it has at least one trick you don't find in a typical regular expression package: a way to match balanced parenthesis. But it has its limits as well.
When those limits are exceeded, then I reach for LPeg. LPeg is an implementation of a Parsing Expression Grammer for Lua, and was implemented by one of Lua's original authors so the adaptation to Lua is done quite well. A PEG allows specification of anything from simple patterns through complete language grammars to be written. LPeg compiles the grammar to a bytecode and executes it extremely efficiently.
you should NOT be trying to parse HTML with regular expressions, HTML and XML are NOT regular languages and can not be successfully manipulated with regular expressions. You should use a dedicated HTML parser. Here are lots of explanations why.