I know that I'd likely hear "Don't parse HTML with regex", so let me say that this question is just academic at this point because I actually solved my problem using the DOM, but on my road to a solution, I ran across this pattern that works on the gskinner website, but I can't figure out how to make it work in PHP preg_match().
(?<=href\=")[^]+?(?=")
I think that the [^] is causing the problem, but I'm not certain what to do about it.
What it is intended to do is pull the substring from between the quotes of an href. (One would expect it to be a web-address or at least part of one.)
[^] is a difficult construct. Basically it is an empty negated character class. But what should it match? That depends on the implementation. Some languages are interpreting it as negation of nothing, so it will match every character, that is what gskinner (means ActionScript 3) seems to be doing.
I would never use this, because it is ambiguous.
The most readable way is to use ., the meta character that matches every character (without newlines), if newlines are also wanted, just add the modifier s that enables the dotall mode, this would be exactly what you wanted to achieve with [^].
A workaround that is sometimes used is to use a character class something like this [\s\S] or [\w\W]. Those will also match every character (including newlines), because they are matching some predefined character class and their negation.
Related
I want to match any full Unicode character. I'm probably using the wrong terms, but I don't necessarily mean letters; I want any displayed character with any modifiers included. Edit: I'm keeping my original wording, but upon review of this answer, perhaps grapheme is actually what I'm looking for.
Using the trivial regex ., with the Unicode u modifier, /./u does not fully suffice. A few examples:
β€οΈ will instead match β€ without the variation selector U+FE0F.
π§π» will only match π§ without the pale skin tone U+1F3Fb.
aΜ (U+0061 (a) followed by U+0300 (grave accent)) will only match the a.
Following this answer, I was able to expand the pattern to this: /.[\x{1f3fb}-\x{1f3ff}\p{M}]?/u. This matches all of my test characters above, as well as the three han unification characters I pulled from this web page.
Edit: I just realized this still doesn't fully match, because (at least in PHP) it fails to fully match ππ½ββ (might not display properly on all devices), because it doesn't capture the male character U+2642.
At this point, it seems like a guessing game to me. I have a feeling there are a lot of edge cases my current regex will not cover, but I don't know enough about foreign alphabets nor am I ready to just start guessing and enumerating random emojis and symbols from the character map to fully test this.
Is there a simpler solution to actually match any character including its modifiers/combining marks/etc?
Edit: Per Rob's comment below, I'm using PHP 7.4 for the regex.
I have a regex that will be used to match #users tags.
I use lokarround assertions, letting punctuation and white space characters surround the tags.
There is an added complication, there are a type of bbcodes that represent html.
I have two types of bbcodes, inline (^B bold ^b) and blocks (^C center ^c).
The inline ones have to be passed thru to reach for the previous or next character.
And the blocks are allowed to surround a tag, just like punctuation.
I made a regex that does work. What I want to do now is to lower the number of steps that it does in every character thatβs not going to be a match.
At first I thought I could do a regex that would just look for #, and when found, it would start looking at the lookarrounds, that worked without the inline bbcodes, but since lookbehind cannot be quantifiable, itβs more difficult since I cannot add ((\^[BIUbiu])++)* inside, producing much more steps.
How could I do my regex more efficient with fewer steps?
Here is a simplified version of it, in the Regex101 link there is the full regex.
(?<=[,\.:=\^ ]|\^[CJLcjl])((\^[BIUbiu])++)*#([A-Za-z0-9\-_]{2,25})((\^[BIUbiu])++)*(?=[,\.:=\^ ]|\^[CJLcjl])
https://regex101.com/r/lTPUOf/4/
A rule of thumb:
Do not let engine make an attempt on matching each single one character if
there are some boundaries.
The quote originally comes from this answer. Following regular expression reduces steps in a significant manner because of the left side of the outermost alternation, from ~20000 to ~900:
(?:[^#^]++|[#^]{2,}+)(*SKIP)(*F)
|
(?<=([HUGE-CHARACTER-CLASS])|\^[cjleqrd])
(\^[34biu78])*+#([a-z\d][\w-.]{0,25}[a-z\d])(\^[34biu78])*+(?=(?1))
Actually I don't care much about the number of steps being reported by regex101 because that wouldn't be true within your own environment and it is not obvious if some steps are real or not or what steps are missed. But in this case since the logic of regex is clear and the difference is a lot it makes sense.
What is the logic?
We first try to match what probably is not desired at all, throw it away and look for parts that may match our pattern. [^#^]++ matches up to a # or ^ symbols (desired characters) and [#^]{2,}+ prevents engine to take extra steps before finding out it's going nowhere. So we make it to fail as soon as possible.
You can use i flag instead of defining uppercase forms of letters (this may have a little impact however).
See live demo here
For example, if I wanted to match text with multiple lines I could use the /s modifier in preg_match.
Or I could use a character class like [^!]+ instead of the .+. (assuming I didn't have any exclamation points in my RegEx)
Problem is there might be an exclamation mark sometimes. Also, when I do this it is greedy and matches all the way to the end.
Sorry for the newbie question but I can't test /s in http://regexpal.com/ and I really like its interface. Basically I want a character class that won't be used in the text and one that isn't greedy so it doesn't try to go as far as it can.
Thanks!
What about using
(.|\n)
That should explicitely allow newlines, too.
I'm getting insane over this, it's so simple, yet I can't figure out the right regex. I need a regex that will match blacklisted words, ie "ass".
For example, in this string:
<span class="bob">Blacklisted word was here</span>bass
I tried that regex:
((?!class)ass)
That matches the "ass" in the word "bass" bot NOT "class".
This regex flags "ass" in both occurences. I checked multiple negative lookaheads on google and none works.
NOTE: This is for a CMS, for moderators to easily find potentially bad words, I know you cannot rely on a computer to do the filtering.
If you have lookbehind available (which, IIRC, JavaScript does not and that seems likely what you're using this for) (just noticed the PHP tag; you probably have lookbehind available), this is very trivial:
(?<!cl)(ass)
Without lookbehind, you probably need to do something like this:
(?:(?!cl)..|^.?)(ass)
That's ass, with any two characters before as long as they are not cl, or ass that's zero or one characters after the beginning of the line.
Note that this is probably not the best way to implement a blacklist, though. You probably want this:
\bass\b
Which will match the word ass but not any word that includes ass in it (like association or bass or whatever else).
It seems to me that you're actually trying to use two lists here: one for words that should be excluded (even if one is a part of some other word), and another for words that should not be changed at all - even though they have the words from the first list as substrings.
The trick here is to know where to use the lookbehind:
/ass(?<!class)/
In other words, the good word negative lookbehind should follow the bad word pattern, not precede it. Then it would work correctly.
You can even get some of them in a row:
/ass(?<!class)(?<!pass)(?<!bass)/
This, though, will match both passhole and pass. ) To make it even more bullet-proof, we can add checking the word boundaries:
/ass(?<!\bclass\b)(?<!\bpass\b)(?<!\bbass\b)/
UPDATE: of course, it's more efficient to check for parts of the string, with (?<!cl)(?<!b) etc. But my point was that you can still use the whole words from whitelist in the regex.
Then again, perhaps it'd be wise to prepare the whitelists accordingly (so shorter patterns will have to be checked).
Is this one is what you want ? (?<!class)(\w+ass)
1. (.*?)
2. (*)
3. #regex#
4. /regex/
A. What do the above symbols mean?
B. What is the different between # and /?
I have the cheat-sheet, but didn't full get it yet. What i know * gets
all characters, so what .*? is for!
The above patterns are used in PHP preg_match and preg_replace.
. matches any character (roughly).
*? is a so-called quantifier, matching the previous token at least zero times (and only as often as needed to complete a match β it's lazy, hence the ?).
(...) create a capturing group you can refer to in either the regex or the match. They also are used for limiting the reach of the | alternation to only parts of the regex (just like parentheses in math make precedence clear).
/.../ and #...# are delimiters for the entire regex, in PHP at least. Technically they're not part of the regex syntax. What delimiter you use is up to you (but I think you can't use \), and mostly changes what characters you need to escape in the regex. So / is a bad choice when you're matching URIs that might contain a lot of slashes. Compare the following two varaints for finding end-of-line comments in C++ style:
preg_match('/\/\/.*$/', $text);
preg_match('#//.*$#', $text);
The latter is easier to read as you don't have to escape slashes within the regex itself. # or # are commonly used as delimiter because they stands out and aren't that frequent in text, but you can use whatever you like.
Technically you don't need this delimiter at all. This is probably mostly a remnant of PHP's Perl heritage (in Perl regexes are delimited, but are not contained in a string). Other languages that use strings (because they have no native regex literals), such as Java, C# or PowerShell do well without the delimiter. In PHP you can add options after the closing delimiter, such as /a/i which matches a or A (case-insensitively), but the regex (?i)a does exactly the same and doesn't need delimiters.
And next time you take the time to read through Regular-Expressions.info, it's an awesome reference on regex basics and advcanced topics, explaining many things very well and thoroughly. And please also take a look at the PHP documentation in this regard.
Well, please stick to one actual question per ... question.
This is an answer to question 3+4, as the other questions have allready been answered.
Regexpes are generally delimited by /, e.g. /abc123/ or /foo|bar/i. In php, you can use whatever character for this you want. You are not limited to /, i.e. you can use e.g. # or %, #/usr/local/bin#.