I am extracting the tags and their surrounding text in the HTML source code. This is the Regex pattern that I use in my code:
String a_pattern = "(.*?)<a (.*?)</a>(.*)";
I tried to use "?" to make the quantifier lazy. However, this pattern takes lots of time while compiling a long string.
Can you please give me some hints on optimizing this pattern?
I should note that I need all three groups of text (before, within, and after )
Thank you
If you want only to get the content from those tags you can use this regex:
<a.*?>(.*?)<\/a>
Working demo
You can check the performance for both regex in this link:
Regex performance
As you can see, the regex you are using is 97% slower.
This result uses the javascript regex engine, but it is really useful to know regex performance.
Related
I'm upgrading a Symfony app with VSCode and I have numerous occurences of this kind of string :
#Template("Area:Local:delete.html.twig")
or
#Template("Group:add.html.twig")
In this case, I want to replace all the : with / to have :
#Template("Area/Local/delete.html.twig")
I can't think of doing it manually, so I was looking for a regular expression for a search/replace in the editor.
I've been toying with this fearsome beast without luck (i'm really dumb in regexp) :
#Template\("([:]*)"
#Template\("(.*?)"
#Template\("[a-zA-Z.-]{0,}[:]")
Still, I think there should be a "simple" regexp for this kind of standard replacement.
Anyone has any clue ? Thank you for any hint
You can use this regex with a capture group: (#Template.*):.
And replace with this $1/.
But you'll have to use replace all until there's no : left, that won't take long.
Just explaining a lit bit more, everything between the parenthesis is a capture group that we can reference later in replace field, if we had (tem)(pla)te, $1 would be tem and $2 would be pla
Regex!
You can use this regex #Template\("(.[^\(\:]*)?(?:\:)(.[^\(\:]*)?(?:\:)?(.[^\(\:]*)?"\) and replacement would simply be #Template\("$1/$2/$3
You can test it out at https://regex101.com/r/VfZHFa/2
Explanation: The linked site will give a better explanation than I can write here, and has test cases you can use.
I want here to submit a very specific performance problem that i want to understand.
Goal
I'm trying to validate a custom synthax with a regex. Usually, i'm not encountering performance issues, so i like to use it.
Case
The regex:
^(\{[^\][{}(),]+\}\s*(\[\s*(\[([^\][{}(),]+\s*(\(\s*([^\][{}(),]+\,?\s*)+\))?\,?\s*)+\]\s*){1,2}\]\s*)*)+$
A valid synthax:
{Section}[[actor1, actor2(syno1, syno2)][expr1,expr2]][[actor3,actor4(syno3, syno4)][expr3,expr4]]
You could find the regex and a test text here :
https://regexr.com/3jama
I hope that be sufficient enough, i don't know how to explain what i want to match more than with a regex ;-).
Issue
Applying the regex on valid text is not costing much, it's almost instant.
But when it comes to specific not valid text case, the regexr app hangs. It's not specific to regexr app since i also encountered dramatic performances with my own java code or javascript code.
Thus, my needs is to validate all along the user is typing the text. I can even imagine validating the text on click, but i cannot afford that the app will be hanging if the text submited by the user is structured as the case below, or another that produce the same performance drop.
Reproducing the issue
Just remove the trailing "]" character from the test text
So the invalid text to raise the performance drop becomes:
{Section}[[actor1, actor2(syno1, syno2)][expr1,expr2]][[actor3,actor4(syno3, syno4)][expr3,expr4
Another invalid test could be, and with no permformance drop:
{Section}[[actor1, actor2(syno1, syno2)][expr1,expr2]][[actor3,actor4(syno3, syno4)][expr3,expr4]]]
Request
I'll be glad if a regex guru coming by could explain me what i'm doing wrong, or why my use case isn't adapted for regex.
This answer is for the condensed regex from your comment:
^(\{[^\][{}(),]+\}(\[(\[([^\][{}(),]+(\(([^\][{}(),]+\,?)+\))?\,?)+\]){1,2}\])*)+$
The issues are similar for your original pattern.
You are facing catastrophic backtracking. Whenever the regex engine cannot complete a match, it backtracks into the string, trying to find other ways to match the pattern to certain substrings. If you have lots of ambiguous patterns, especially if they occur inside repetitions, testing all possible variations takes a looooong time. See link for a better explanation.
One of the subpatterns that you use is the following (multilined for better visualisation):
([^\][{}(),]+
(\(
([^\][{}(),]+\,?)+
\))?
\,?)+
That is supposed to match a string like actor4(syno3, syno4). Condensing this pattern a little more, you get to ([^\][{}(),]+,?)+. If you remove the ,? from it, you get ([^\][{}(),]+)+ which is an opening gate to the catasrophic backtracking, as string can be matched in quite a lot of different ways with this pattern.
I get what you try to do with this pattern - match an identifier - and maybe other other identifiers that are separated by comma. The proper way of doing this however is: ([^\][{}(),]+(?:,[^\][{}(),]+)*). Now there isn't an ambiguous way left to backtrack into this pattern.
Doing this for the whole pattern shown above (yes, there is another optional comma that has to be rolled out) and inserting it back to your complete pattern I get to:
^(\{[^\][{}(),]+\}(\[(\[([^\][{}(),]+(\(([^\][{}(),]+(?:,[^\][{}(),]+)*)\))?(?:\,[^\][{}(),]+(\(([^\][{}(),]+(?:,[^\][{}(),]+))*\))?)*)\]){1,2}\])*)+$
Which doesn't catastrophically backtrack anymore.
You might want to do yourself a favour and split this into subpatterns that you concat together either using strings in your actual source or using defines if you are using a PCRE pattern.
Note that some regex engines allow the use of atomic groups and possessive quantifiers that further help avoiding needless backtracking. As you have used different languages in your title, you will have to check yourself, which one is available for your language of choice.
I have a list of the following numbers and want a Regular expression that matches when a number is not in the list.
0,1,2,3,4,9,11,12,13,14,15,16,18,19,250
I have written the following REGEX statement.
^(?!.*(0|1|2|3|4|9|11|12|13|14|15|16|18|19|250)).*$
The problem is that it correctly gives a match for 5,6,7,8 etc but not for 17 or 251 for example.
I have been testing this on the online REGEX simulators.
This should resolve your issue..
^(?!\D*(0|1|2|3|4|9|11|12|13|14|15|16|18|19|250)\b).*$
In your earlier regex you were basically saying eliminate all numbers that start with 0/1/2/3/4/9!
So your original regex would actually match 54/623/71/88 but not the others. Also the 11-19 and 250 in the list were rendered useless.
Although as others have I would also recommend you to not use regex for this, as I believe it is an overkill and a maintenance nightmare!
Also an extra note "Variable length look arounds are very inefficient too" vs regular checks.
I would do \b\d+\b to get each number in the string and check if they are in your list. It would be way faster.
You can use the discard technique by matching what you do not want and capturing what you really want.
You can use a regex like this:
\b(?:[0-49]|1[1-689]|250)\b|(\d+)
Here you can check a working demo where in blue you have the matches (what you don't want) and in green the content you want. Then you have to grab the content from the capturing group
Working demo
Not sure what regex engine you are using, but here I created a sample using java:
https://ideone.com/B7kLe0
I have a code like this:
<pre><code>Some HTML code</code></pre>
I need to escape the HTML between the <pre><code></code></pre> tags. I have lots of tags, so I thought - why not let regex do it for me. The problem is I don't know how. I've seen lots of examples using Google and Stackoverflow, but nothing I could use. Can someone here help me?
Example:
<pre><code>Some HTML code</code></pre>
To
<pre><code>Some <a href="http">HTML</a> code</code></pre>
Or just a regex so I can replace anything between the <pre><code> and </code></pre> tags one by one. I'm almost certain that this can be done.
This regex will match the parts of the anchor tag
you need to put back:
<pre><code>([^<]*?)(.*?)(.*?)</code></pre>
See a live demo, which shows it matching correctly and also shows the various parts being captured as groups which we'll refer to in the replacement string (see below).
Use the regex above with the following replacement:
<pre><code>\1<a href="\2">\3</a>\4</pre></code>
The \1, \2 etc are the captured groups in the regex that put back what we're keeping from the match.
A regular expression to return "the thing between <pre><code> and </code></pre>" could be
/(?<=<pre><code>).*?(?=<\/code><\/pre>)/
This uses lookaround expressions to delimit the "thing that gets matched". Typically using regex in situations with nested tags is fraught with danger and you are much better off using "real tools" made specifically for the job of parsing xml, html etc. I am a huge fan of Beautiful Soup (Python) myself. Not familiar with Notepad++, so not sure if its dialect of regex matches this expression exactly.
I am working on improving our glossary functionality in a custom CMS that is running with classic ASP (ASP 3.0) on IIS with VBScript code. I am stumped on a regex challenge I cannot solve.
Here is the current code:
If InStr(ART_ArticleBody, "href") = False then
sql="SELECT URL, Term, RegX FROM GLOSSARYDB;"
Set rsGlossary = Server.CreateObject("ADODB.Recordset")
rsGlossary.open sql, strSQLConn
Set RegExObject = New RegExp
While Not rsGlossary.EOF
URL = rsGlossary("URL")
Phrase = rsGlossary("RegX")
With RegExObject
.Pattern = Phrase
.IgnoreCase = true
.Global = false
End With
set expressionmatch = RegExObject.Execute(ART_ArticleBody)
if expressionmatch.count > 0 then
For Each expressionmatched in expressionmatch
RegExObject.Pattern = Phrase
URL = ""& expressionmatched.Value & ""
ART_ArticleBody = RegExObject.Replace(ART_ArticleBody, URL)
next
end if
rsGlossary.movenext
wend
rsGlossary.movefirst
Set RegExObject = nothing
end if
Instead of skipping putting glossary links in any article that has an href in it, as the above code does, I would like to change the code to process every article but have the RegEx pattern avoid matching on a glossary entry if the match is inside of an a tag.
For example, in italics below is a test example for this regex entry in my DB: ROI|return on investment|investment return
Here is a link that uses the glossary term: Info on return on investment.
Now, here is the glossary term in plain text, not inside of a link: return on investment.
We want to find the third instance of a match but not find the first two because they are both inside of a HTML link.
In the above text, if I were processing the article for the glossary entry "ROI|return on investment|investment return" I do not want to match on the first or second occurance that match because they are in an a tag. I need the regex pattern to skip over those matches and just match on any that are not inside of an a tag.
Any help on this would be greatly appreciated.
Try this regex:
<a\b[^<>]*>[\s\S]*?</a>|(ROI|return on investment|investment return)
This matches an HTML anchor, or any of the terms you're looking for. The terms are captured into group number 1. So in your VBScript code, check if the first capturing group matched anything, and you've got one of your keywords outside an <a> tag.
This regex indeed won't work correctly if you have nested <a> tags. That shouldn't be a problem, as anchors are normally not nested inside each other. If it is a problem, you can't solve it with VBScript/JavaScript regular expressions. The regex also won't work correctly if you have <a> tags that are missing their closing tags. If you want to take that into account, try this regex:
<a\b[^<>]*>(?:(?:(?!<a\b)[\s\S])*?</a>)?|(ROI|return on investment|investment return)
This problem is, as they say, "non-trivial" in its current state. However, if you could modify your system to output more semantic markup, it would make things much easier:
undesired tag match
This is <span class="tag">a tag</span>
In this case, you can simply search:
(?<=<span class=\"tag\">)(phrase1|phrase2|phrase3)(?=</span>)
Or something a little more robust
(?<=<span class=\"tag\">).+?(?=</span>)
This way you can easily focus your searches to data within a specific <span>, and leave everything else aside.
You can't solve it because it can't be done, at least not with 100% reliability. HTML is not a "regular" language in the regular expression sense. Like the saying goes, when you have a hammer, everything starts to look like a nail. There are some things regular expressions aren't good at. This is one of them.
Most languages have some form of HTML parsing library as standard or easily obtained. Use those. That's what they were designed for.
In general, you can't use a regular expression to recognize arbitrarily nested constructs (such as bracket-delimited HTML tags). If you had solved this problem, there's be a lot of mathematicians lining up to hear about it. :)
Having said that, .NET does indeed offer an extension to regular expressions that permits what I just said was impossible, and--even better!--the sample chapter for the great "Mastering Regular Expressions" available here happens to cover that feature.
(accounts receivable|A/R)(?!((?!</?a\b).)*</a)
(phrase1|phrase2|phrase3)(?!((?!</?a\b).)*</a)
The above approach seems to work, at least in my RegexBuddy software. I didn't figure it out on my own. Had some help from a guru. Time to test it in my ASP code. Thanks to all who provided input. I'm sure I didn't describe what I needed well enough for you to come up with the above solution. Mea culpa.