I would like to match strings/characters that are not surrounded by a well-defined string-wrapper. In this case the wrapper is '#L#' on the left of the string and '#R#' on the right of the string.
With the following string for example:
This is a #L#string#R# and it's #L#good or ok#R# to change characters in the next string
I would like to be able to search for (any number of characters) to change them on a case by case basis. For example:
Searching for "in", would match twice - the word 'in', and the 'in' contained within the last word 'string'.
Searching for a "g", should be found within the word 'change' and in the final word string (but not the first occurrence of string contained within the wrapper).
I'm somewhat familiar with how lookahead works in the sense that it identifies a match, and doesn't return the matching criteria as part of the identified match.
Unfortunately, I can't get my head around how to do it.
I've also been playing with this at http://regexpal.com/ but can't seem to find anything that works. Examples I've found for iOS are problematic, so perhaps the javascript tester is a tiny bit different.
I took some guidance from a previous question I asked, which seemed to be almost the same but sufficiently different to mean I couldn't work out how to reuse it:
Replacing 'non-tagged' content in a web page
Any ideas?
At first all the #L# to #R# blocks and then use alternation operator | to match the string in from the remaining string. To differentiate the matches, put in inside a capturing group.
#L#.*?#R#|(in)
DEMO
OR
Use a negative lookahead assertion. This would match the sub-string in only if it's not followed by #L# or #R#, zero or more times and further followed by #R#. So this would match all the in's which was not present inside the #L# and #R# blocks.
in(?!(?:(?!#[RL]#).)*#R#)
DEMO
Related
I am trying to "highlight" references to law statutes in some text I'm displaying. These references are of the form <number>-<number>-<number>(char)(char), where:
"number" may be whole numbers 18 or decimal numbers 12.5;
the parenthetical terms are entirely optional: zero or one or more;
if a parenthetical term does exist, there may or may not be a space between the last number and the first parenthesis, as in 18-1.3-401(8)(g) or 18-3-402 (2).
I am using the regex
((\d+(\.\d+)*-){2}(\d+(\.\d+)*))( ?(\([0-9a-zA-Z]+\))*)
to find the ranges of these strings and then highlight them in my text. This expression works perfectly, 100% of the time, in all of the cases I've tried (dozens), in BBEdit, and on regex101.com and regexr.com.
However, when I use that exact same expression in my code, on iOS 12.2, it is extremely hit-or-miss as to whether a string matching the regex is actually found. So hit-or-miss, in fact, that a string of the exact same form of two other matches in a specific bit of text is NOT found. E.g., in this one paragraph I have, there are five instances of xxx-x-xxx; the first and the last are matched, but the middle three are not matched. This makes no sense to me.
I'm using the String method func range(of:options:range:locale:) with options of .regularExpression (and nil locale) to do the matching. I see that iOS uses ICU-compatible regexes, whereas these other tools use PCRE (I think). But, from what I can tell, my expression should be compatible and valid for my case with the ICU parsing. But, something is definitely different, and I cannot figure out what it is.
Anyone? (I'm going to give NSRegularExpression a go and see if it behaves differently, but I'd still like to figure out what's going on here.)
I'm working on a Guitar Chord transposer, and so from a given text file, I want to identify guitar chords. e.g. G#, Ab, F#m, etc.
I'm almost there! I have run into a few problems already due to the number sign (hash tag).
#
For example, you can't include the number sign in your regex pattern. The NSRegularExpression will not initialize with this:
let fail: String = "\\b[ABCDEFG](b|#)?\\b"
let success: String = "\\b[CDEFGAB](b|\\u0023)?\\b"
I had to specifically provide the unicode character. I can live with that.
However, now that I have a NSRegularExpression object, it won't match these (sharps = number sign) when I have a line of text such as:
Am Bb G# C Dm F E
When it starts processing the G#, the sharp associated with that second capture group is not matched. (i.e. the NSTextCheckingResult's second range has a location of NSNotFound) Note, it works for Bb... it matches the 'b'
I'm wondering what I need to do here. It would seem the documentation doesn't cover this case of '#' which IS in fact sometimes used in Regex patterns (I think related to comments or sth)
One thing that would be great would be to not have to look up the unicode identifier for a #, but just use it as a String "#" then convert that so it plays nicely with the pattern. There exists the chance that \u0023 is in fact not the code associated with # ...
The \b word boundary is a context dependent construct. It matches in 4 contexts: 1) between start of string and a word char, 2) between a word char and end of string, 3) between word and a non-word and 4) a non-word and a word char.
Your regex is written in such a way that ultimately the regex engine sees a \b after # and that means a # will only match if there is a word char after it.
If you replace \b with (?!\w), a negative lookahead that fails the match if there is a word char immediately to the right of the current location, it will work.
So, you may use
\\b[CDEFGAB](b|\\u0023)?(?!\\w)
See the regex demo.
Details
\b - a word boundary
[CDEFGAB] - a char from the set
(b|\\u0023)? - an optional sequence of b or #
(?!\\w) - a negative lookahead failing the match (and causing backtracking into the preceding pattern! To avoid that, add + after ? to prevent backtracking into that pattern) if there is a word char immediately to the right of the current position.
(I'd like to first say #WiktorStribiżew has been a tremendous help and what I am writing now would not have been possible without him! I'm not concerned about StackOverflow points and rep, so if you like this answer, please upvote his answer.)
This issue took many turns and had a few issues going on. Ultimately this question should be called How do I use Regex on iOS to detect Musical Chords in a text file?
The answer is (so far), not simply.
CRASH COURSE IN MUSIC THEORY
In music you have notes. They are made up of a letter between A->G and an optional symbol called an accidental. (A note relates to the acoustic frequency of the sound you hear when that note is played) An accidental can be a flat (represented as a ♭ or simply a b), or a sharp (represented as a ♯ or simply a #, as these are easier to type on a keyboard). An accidental serves to make a note a semitone higher (#) or lower (b). As such, a F# is the same acoustic frequency as a Gb. On a piano, the white keys are notes with no accidentals, and the black keys represent notes with an accidental. Depending on some factors of the piece of music, that piece won't mix accidental types. It will either be flats throughout the piece or sharps. (Depending on the musical key of the composition, but this is not that relevant here.)
In terms of regex, you have something like ABCDEFG? to determine the note. In reality it's more complicated.
Then, a Musical Chord is comprised of the root note and it's chord type. There are over 50 types of chords. They have a 'text signature' that is unique. Also, a 'major' chord has an empty signature. So in terms of pseudo-regex you have for a Chord:
[ABCDEFG](b|#)?(...|...|...)?
where the first part you recognize as the note (as before), and the last optional is to determine the chord type. The different types were omitted, but can be as simple as a m (for Minor chord), or maj7#5 (for a major 7th chord with an augmented 5th... don't worry about it. Just know there are many string constants to represent a chord type)
Then finally, with guitar you often have a corresponding bass note that changes the chord's tonality somewhat. You denote this by adding a slash and then the note, giving the general pseudoform:
[ABCDEFG](b|#)?(...|...|...)?(/[ABCDEFG](b|#)?)? // NOT real Regex
real examples: C/F or C#m/G# and so on
where the last part has a slash then the same pattern to recognize a note.
So putting these all together, in general we want to find chords that could take on many forms, such as:
F Gm C#maj7/G# F/C Am A7 A7/F# Bmaj13#11
I was hoping to find one Regex to rule them all. I ended up writing code that works, though it seems like I kind of hacked around a bit to get the results I desired.
You can see this code here, written in Swift. It is not complete for my purposes, but it will parse a string, return a list of Chord Results and their text range within the original string. From there you would have to finish the implementation to suit your needs.
There have been a few issues on iOS:
iOS does not handle the number sign (#) well at all. When providing regex patterns or match text, I either had to replace the # with its unicode \u0023, or what ultimately worked was replacing all occurrences of # with another character (such as 'S'), and then convert it back once regex did it's thing. So this code I wrote often has to 'sanitize' the pattern or the input text before doing anything.
I couldn't get a Regex Pattern to perfectly parse a chord structure. It wasn't fully working for a Chord with a bass note, but it would successfully match a Chord with a bass note, then I had to split those 2 components and parse them separately, then recombine them
Regex is really a bit of voodoo, and I think it sucks that for something so confusing to many people, there are also different platform-dependent implementations of it. For example, Wiktor referred me to Regex patterns he wrote to help me solve the problem on www.regex101.com, that would WORK on that website, but these would not work on iOS, and NSRegularExpression would throw an error (often it had something to do with this # character)
My solution pays absolutely no regard to performance. It just wanted it to work.
I have 2D array in which the second column has domain names of some emails, let us call the array myData[][]. I decided to use ArrayLib in order to search the second column for a specific domain.
ArrayLib.indexOf(myData, 1, domain)
Here is where I found an issue. In myData array, one of the domains look like this "ewmining.com" (pay attention to the w).
While searching for "e.mining.com" (notice the first dot), the indexOf() function actully gave me the row containing "ewmining.com".
This is what is in the array "ewmining.com"
This is what is in the serach string "e.mining.com"
It seams that ArrayLib treats the dot to mean any character. Is this supposed to be the correct behavior? Is there a way to stop this behavior and search for exact match.
I really need help on this issue.
Thanks in advance for your help.
The dot usually represents "any character" in regular expressions. I am not familiar with ArrayLib, but maybe you should look for a way to turn off regular expressions when searching. Otherwise you might have to escape the dot, for example search for e[.]mining[.]com
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.
Question
I would like to be able to use a single regex (if possible) to require that a string fits [A-Za-z0-9_] but doesn't allow:
Strings containing just numbers or/and symbols.
Strings starting or ending with symbols
Multiple symbols next to eachother
Valid
test_0123
t0e1s2t3
0123_test
te0_s1t23
t_t
Invalid
t__t
____
01230123
_0123
_test
_test123
test_
test123_
Reasons for the Rules
The purpose of this is to filter usernames for a website I'm working on. I've arrived at the rules for specific reasons.
Usernames with only numbers and/or symbols could cause problems with routing and database lookups. The route for /users/#{id} allows id to be either the user's id or user's name. So names and ids shouldn't be able to collide.
_test looks wierd and I don't believe it's valid subdomain i.e. _test.example.com
I don't like the look of t__t as a subdomain. i.e. t__t.example.com
This matches exactly what you want:
/\A(?!_)(?:[a-z0-9]_?)*[a-z](?:_?[a-z0-9])*(?<!_)\z/i
At least one alphabetic character (the [a-z] in the middle).
Does not begin or end with an underscore (the (?!_) and (?<!_) at the beginning and end).
May have any number of numbers, letters, or underscores before and after the alphabetic character, but every underscore must be separated by at least one number or letter (the rest).
Edit: In fact, you probably don't even need the lookahead/lookbehinds due to how the rest of the regex works - the first ?: parenthetical won't allow an underscore until after an alphanumeric, and the second ?: parenthetical won't allow an underscore unless it's before an alphanumeric:
/\A(?:[a-z0-9]_?)*[a-z](?:_?[a-z0-9])*\z/i
Should work fine.
I'm sure that you could put all this into one regular expression, but it won't be simple and I'm not sure why insist on it being one regex. Why not use multiple passes during validation? If the validation checks are done when users create a new account, there really isn't any reason to try to cram it into one regex. (That is, you will only be dealing with one item at a time, not hundreds or thousands or more. A few passes over a normal sized username should take very little time, I would think.)
First reject if the name doesn't contain at least one number; then reject if the name doesn't contain at least one letter; then check that the start and end are correct; etc. Each of those passes could be a simple to read and easy to maintain regular expression.
What about:
/^(?=[^_])([A-Za-z0-9]+_?)*[A-Za-z](_?[A-Za-z0-9]+)*$/
It doesn't use a back reference.
Edit:
Succeeds for all your test cases. Is ruby compatible.
This doesn't block "__", but it does get the rest:
([A-Za-z]|[0-9][0-9_]*)([A-Za-z0-9]|_[A-Za-z0-9])*
And here's the longer form that gets all your rules:
([A-Za-z]|([0-9]+(_[0-9]+)*([A-Za-z|_[A-Za-z])))([A-Za-z0-9]|_[A-Za-z0-9])*
dang, that's ugly. I'll agree with Telemachus, that you probably shouldn't do this with one regex, even though it's technically possible. regex is often a pain for maintenance.
The question asks for a single regexp, and implies that it should be a regexp that matches, which is fine, and answered by others. For interest, though, I note that these rules are rather easier to state directly as a regexp that should not match. I.e.:
x !~ /[^A-Za-z0-9_]|^_|_$|__|^\d+$/
no other characters than letters, numbers and _
can't start with a _
can't end with a _
can't have two _s in a row
can't be all digits
You can't use it this way in a Rails validates_format_of, but you could put it in a validate method for the class, and I think you'd have much better chance of still being able to make sense of what you meant, a month or a year from now.
Here you go:
^(([a-zA-Z]([^a-zA-Z0-9]?[a-zA-Z0-9])*)|([0-9]([^a-zA-Z0-9]?[a-zA-Z0-9])*[a-zA-Z]+([^a-zA-Z0-9]?[a-zA-Z0-9])*))$
If you want to restrict the symbols you want to accept, simply change all [^a-zA-Z0-9] with [] containing all allowed symbols
(?=.*[a-zA-Z].*)^[A-Za-z0-9](_?[A-Za-z0-9]+)*$
This one works.
Look ahead to make sure there's at least one letter in the string, then start consuming input. Every time there is an underscore, there must be a number or a letter before the next underscore.
/^(?![\d_]+$)[A-Za-z0-9]+(?:_[A-Za-z0-9]+)*$/
Your question is essentially the same as this one, with the added requirement that at least one of the characters has to be a letter. The negative lookahead - (?![\d_]+$) - takes care of that part, and is much easier (both to read and write) than incorporating it into the basic regex as some others have tried to do.
[A-Za-z][A-Za-z0-9_]*[A-Za-z]
That would work for your first two rules (since it requires a letter at the beginning and end for the second rule, it automatically requires letters).
I'm not sure the third rule is possible using regexes.