We have a configuration file that lists a series of regular expressions used to exclude files for a tool we are building (it scans .class files). The developer has appended all of the individual regular expressions into a single one using the OR "|" operator like this:
rx1|rx2|rx3|rx4
My gut reaction is that there will be an expression that will screw this up and give us the wrong answer. He claims no; they are ORed together. I cannot come up with case to break this but still fee uneasy about the implementation.
Is this safe to do?
Not only is it safe, it's likely to yield better performance than separate regex matching.
Take the individual regex patterns and test them. If they work as expected then OR them together and each one will still get matched. Thus, you've increased the coverage using one regex rather than multiple regex patterns that have to be matched individually.
As long as they are valid regexes, it should be safe. Unclosed parentheses, brackets, braces, etc would be a problem. You could try to parse each piece before adding it to the main regex to verify they are complete.
Also, some engines have escapes that can toggle regex flags within the expression (like case sensitivity). I don't have enough experience to say if this carries over into the second part of the OR or not. Being a state machine, I'd think it wouldn't.
It's as safe as anything else in regular expressions!
As far as regexes go , Google code search provides regexes for searches so ... it's possible to have safe regexes
I don't see any possible problem too.
I guess by saying 'Safe' you mean that it will match as you needed (because I've never heard of RegEx security hole). Safe or not, we can't tell from this. You need to give us more detail like what the full regex is. Do you wrap it with group and allow multiple? Do you wrap it with start and end anchor?
If you want to match a few class file name make sure you use start and end anchor to be sure the matching is done from start til end. Like this "^(file1|file2)\.class$". Without start and end anchor, you may end up matching 'my_file1.class too'
The answer is that yes this is safe, and the reason why this is safe is that the '|' has the lowest precedence in regular expressions.
That is:
regexpa|regexpb|regexpc
is equivalent to
(regexpa)|(regexpb)|(regexpc)
with the obvious exception that the second would end up with positional matches whereas the first would not, however the two would match exactly the same input. Or to put it another way, using the Java parlance:
String.matches("regexpa|regexpb|regexpc");
is equivalent to
String.matches("regexpa") | String.matches("regexpb") | String.matches("regexpc");
Related
I'm trying to build a regular expression and replacement string that I can use to insert missing apostrophes. Examples:
Dont -> Don't
Ill -> I'll
I can get this working with capture groups, but I'm trying to only have to call .Replace one time. Right now I have something like:
$apostropheregex = '\b((didn|won|ain|don)(t)|(i)(ll|m))\b'
$apostrophereplacement='$2$4''$3$5'
But it feels ugly to be mashing together both prefix groups and both postfix groups with the assumption that we only matched one or the other (either a "ll" or a "t" match)
Does anyone have any suggestions? Is there a better way to approach this problem? Should I indeed treat these as two separate scenarios and run replace twice with separate regexes and replacement strings?
Update: To clarify, I'm aware that this could have unintended consequences, replacing strings that shouldn't be replaced since English grammatical context is not considered. I'm running this manually after reviewing strings first and I still think this is an interesting question.
Just a note: This is ill suited for... ill suited, which becomes i'll suited.
But you asked for a better regex and you shall receive. I would use:
\b(?|(don)(t)|(won)(t)|(you)(re))\b
The replacement will be $1'$2.
The main advantage of this regex is legibility. You should easily be able to add new alterations.
It works by using the branch reset group (?| ). This means that each alternation uses $1 and $2 (instead of 2n+1 and 2n+2).
I'm pulling car submodels from the DB and I'm building my regular expression on the fly.
Here is an example of a search string:
EX-L Sedan 4-Door
Here is my regular expression:
preg_match("/LX|EX|EX-L|LX-P|LX-S/Ui", $input_line, $output_array);
For some reason the output is EX and not EX-L as it supposed to be. Can someone explain why?
Your pattern is unanchored and thus the first alternative that matches a substring makes the regex engine stop processing the whole group. This is a common behavior with NFA regexes.
Also, there are no quantifiers in your pattern, thus the /U modifier is redundant.
So, you can use
/EX-L|LX-P|LX-S|LX|EX/i
It is a readable form. However, best practice with regexes is to make sure no alternative branch can match at the same location as another. That means you can use
/EX(-L)?|LX(-[PS])?/i
As others have pointed out, the reason for this undesired outcome is because the regex engine is happy to have the first alternative and run for the door since your pattern has no anchors (like: ^, $, and some other lesser known ones). This is the same short-circuiting behavior you'd see in php's if($x || $y) conditions; if $x is true there is no need to evaluate further. But enough about that...
I would like to offer some additional logic that I think is relevant to your case/question.
You say your regex is built on the fly, so I am assuming your method goes something like this:
A user identifies which substrings/keywords they want to search for.
$strings=array('LX','EX','EX-L','LX-P','LX-S');
// array of substrings in any order
As mentioned earlier, you need longer strings to precede shorter ones with identical starting characters.
rsort($strings);
// sort DESC, longer strings precede shorter strings when leading characters match
Pipe all strings into a single regex pattern with implode().
$piped_regex='/\b(?:'.implode('|',$array).')\b/i';
// word boundaries ensure the string is not part of a larger word; remove if not desired
// pattern: /\b(?:LX-S|LX-P|LX|EX-L|EX)\b/i
While programmatically condensing your similar strings into a concise pattern as Wiktor recommended is possible, it's probably not worth the effort with your on-the-fly patterns.
Finally run preg_match() as normal.
$input_line='EX-L Sedan 4-Door';
if(preg_match($piped_regex,$input_line,$output_array)){
var_export($output_array);
}
// output: array(0=>'EX-L')
I hope stepping out this method is helpful to you and future SO readers.
I have scoured the web in the past few hours trying to figure out why in the world one of my colleagues insists on using (?!.) as a last-character in his regular expressions instead of the usual $.
Some of the regular expressions I've seen have been ^.*.txt(?!.) which begin with the usual ^, but do not end with the $. I have not been able to find any definitive or time-efficient reasons, any pros and cons or differences at all?
$ may match end of line rather than end of input (this depends on modifiers used). Perhaps this is the reason.
In my opinion, the best way to match the end of input is \z - which means exactly end of input, regardless of modifiers. It is supported in most (if not all) regex implementations.
The only possible difference is with multiline
asdf$ :
http://rubular.com/r/B2cNEL1pln
asdf(?!.) :
http://rubular.com/r/rbhKi1lKGI
^.*\.txt(?!.) means match (beginning)(anything 0 or more times).txt and is not followed by anything.
You can get more info on the ?! pattern here.
If you look here, it says that using the m or s modifiers, you can modify the behavior of ^ and $, to match beginning or end of line, rather than the whole string. There's also an ms. So, I guess with (?!.), you can match the end of the entire multi-line string.
So, I wouldn't say using this is better. Rather, I would say you need to know exactly what you're looking for or what you actually intend to do, within a single-lined string, or multi-lined string and how you want to parse your input to get one-line or multi-line strings, before passing into the regexp.
I think many of us run regexps on single-lined strings and therefore do not feel a difference between the two syntaxes.
I guess this is more or less a two-part question, but here's the basics first: I am writing some PHP to use preg_match_all to look in a variable for strings book-ended by {}. It then iterates through each string returned, replaces the strings it found with data from a MySQL query.
The first question is this: Any good sites out there to really learn the ins and outs of PCRE expressions? I've done a lot of searching on Google, but the best one I've been able to find so far is http://www.regular-expressions.info/. In my opinion, the information there is not well-organized and since I'd rather not get hung up having to ask for help whenever I need to write a complex regex, please point me at a couple sites (or a couple books!) that will help me not have to bother you folks in the future.
The second question is this: I have this regex
"/{.*(_){1}(.*(_){1}[a-z]{1}|.*)}/"
and I need it to catch instances such as {first_name}, {last_name}, {email}, etc. I have three problems with this regex.
The first is that it sees "{first_name} {last_name}" as one string, when it should see it as two. I've been able to solve this by checking for the existence of the space, then exploding on the space. Messy, but it works.
The second problem is that it includes punctuation as part of the captured string. So, if you have "{first_name} {last_name},", then it returns the comma as part of the string. I've been able to partially solve this by simply using preg_replace to delete periods, commas, and semi-colons. While it works for those punctuation items, my logic is unable to handle exclamation points, question marks, and everything else.
The third problem I have with this regex is that it is not seeing instances of {email} at all.
Now, if you can, are willing, and have time to simply hand me the solution to this problem, thank you as that will solve my immediate problem. However, even if you can do this, please please provide an lmgfty that provides good web sites as references and/or a book or two that would provide a good education on this subject. Sites would be preferable as money is tight, but if a book is the solution, I'll find the money (assuming my local library system is unable to procure said volume).
Back then I found PHP's own PCRE syntax reference quite good: http://uk.php.net/manual/en/reference.pcre.pattern.syntax.php
Let's talk about your expression. It's quite a bit more verbose than necessary; I'm going to simplify it while we go through this.
A rather simpler way of looking at what you're trying to match: "find a {, then any number of letters or underscores, then a }". A regular expression for that is (in PHP's string-y syntax): '/\{[a-z_]+\}/'
This will match all of your examples but also some wilder ones like {__a_b}. If that's not an option, we can go with a somewhat more complex description: "find a {, then a bunch of letters, then (as often as possible) an underscore followed by a bunch of letters, then a }". In a regular expression: /\{([a-z]+(_[a-z]+)*\}/
This second one maybe needs a bit more explanation. Since we want to repeat the thing that matches _foo segments, we need to put it in parentheses. Then we say: try finding this as often as possible, but it's also okay if you don't find it at all (that's the meaning of *).
So now that we have something to compare your attempt to, let's have a look at what caused your problems:
Your expression matches any characters inside the {}, including } and { and a whole bunch of other things. In other words, {abcde{_fgh} would be accepted by your regex, as would {abcde} fg_h {ijkl}.
You've got a mandatory _ in there, right after the first .*. The (_){1} (which means exactly the same as _) says: whatever happens, explode if this ain't here! Clearly you don't actually want that, because it'll never match {email}.
Here's a complete description in plain language of what your regex matches:
Match a {.
Match a _.
Match absolutely anything as long as you can match all the remaining rules right after that anything.
Match a _.
Match a single letter.
Instead of that _ and the single letter, absolutely anything is okay, too.
Match a }.
This is probably pretty far from what you wanted. Don't worry, though. Regular expressions take a while to get used to. I think it's very helpful if you think of it in terms of instructions, i.e. when building a regular expression, try to build it in your head as a "find this, then find that", etc. Then figure out the right syntax to achieve exactly that.
This is hard mainly because not all instructions you might come up with in your head easily translate into a piece of a regular expression... but that's where experience comes in. I promise you that you'll have it down in no time at all... if you are fairly methodical about making your regular expressions at first.
Good luck! :)
For PCRE, I simply digested the PCRE manpages, but then my brain works that way anyway...
As for matching delimited stuff, you generally have 2 approaches:
Match the first delimiter, match anything that is not the closing delimiter, match the closing delimiter.
Match the first delimiter, match anything ungreedily, match the closing delimiter.
E.g. for your case:
\{([^}]+)\}
\{(.+?)\} - Note the ? after the +
I added a group around the content you'd likely want to extract too.
Note also that in the case of #1 in particular but also for #2 if "dot matches anything" is in effect (dotall, singleline or whatever your favourite regex flavour calls it), that they would also match linebreaks within - you'd need to manually exclude that and anything else you don't want if that would be a problem; see the above answer for if you want something more like a whitelist approach.
Here's a good regex site.
Here's a PCRE regex that will work: \{\w+\}
Here's how it works:
It's basically looking for { followed by one ore more word characters followed by }. The interesting part is that the word character class actually includes an underscore as well. \w is essentially shorthand for [A-Za-z0-9_]
So it will basically match any combination of those characters within braces and because of the plus sign will only match braces that are not empty.
The Greedy Option of Regex is really needed?
Lets say I have following texts, I like to extract texts inside [Optionx] and [/Optionx] blocks
[Option1]
Start=1
End=10
[/Option1]
[Option2]
Start=11
End=20
[/Option2]
But with Regex Greedy Option, its give me
Start=1
End=10
[/Option1]
[Option2]
Start=11
End=20
Anybody need like that? If yes, could you let me know?
If I understand correctly, the question is “why (when) do you need greedy matching?”
The answer is – almost always. Consider a regular expression that matches a sequence of arbitrary – but equal – characters, of length at least two. The regular expression would look like this:
(.)\1+
(\1 is a back-reference that matches the same text as the first parenthesized expression).
Now let’s search for repeats in the following string: abbbbbc. What do we find? Well, if we didn’t have greedy matching, we would find bb. Probably not what we want. In fact, in most application s we would be interested in finding the whole substring of bs, bbbbb.
By the way, this is a real-world example: the RLE compression works like that and can be easily implemented using regex.
In fact, if you examine regular expressions all around you will see that a lot of them use quantifiers and expect them to behave greedily. The opposite case is probably a minority. Often, it makes no difference because the searched expression is inside guard clauses (e.g. a quoted string is inside the quote marks) but like in the example above, that’s not always the case.
Regular expressions can potentially match multiple portion of a text.
For example consider the expression (ab)*c+ and the string "abccababccc". There are many portions of the string that can match the regular expressions:
(abc)cababccc
(abcc)ababccc
abcc(ababccc)
abccab(abccc)
ab(c)cababccc
ab(cc)ababccc
abcabab(c)ccc
....
some regular expressions implementation are actually able to return the entire set of matches but it is most common to return a single match.
There are many possible ways to determine the "winning match". The most common one is to take the "longest leftmost match" which results in the greedy behaviour you observed.
This is tipical of search and replace (a la grep) when with a+ you probably mean to match the entire aaaa rather than just a single a.
Choosing the "shortest non-empty leftmost" match is the usual non-greedy behaviour. It is the most useful when you have delimiters like your case.
It all depends on what you need, sometimes greedy is ok, some other times, like the case you showed, a non-greedy behaviour would be more meaningful. It's good that modern implementations of regular expressions allow us to do both.
If you're looking for text between the optionx blocks, instead of searching for .+, search for anything that's not "[\".
This is really rough, but works:
\[[^\]]+]([^(\[/)]+)
The first bit searches for anything in square brackets, then the second bit searches for anything that isn't "[\". That way you don't have to care about greediness, just tell it what you don't want to see.
One other consideration: In many cases, greedy and non-greedy quantifiers result in the same match, but differ in performance:
With a non-greedy quantifier, the regex engine needs to backtrack after every single character that was matched until it finally has matched as much as it needs to. With a greedy quantifier, on the other hand, it will match as much as possible "in one go" and only then backtrack as much as necessary to match any following tokens.
Let's say you apply a.*c to
abbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbc. This finds a match in 5 steps of the regex engine. Now apply a.*?c to the same string. The match is identical, but the regex engine needs 101 steps to arrive at this conclusion.
On the other hand, if you apply a.*c to abcbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb, it takes 101 steps whereas a.*?c only takes 5.
So if you know your data, you can tailor your regex to match it as efficiently as possible.
just use this algorithm which you can use in your fav language. No need regex.
flag=0
open file for reading
for each line in file :
if check "[/Option" in line:
flag=0
if check "[Option" in line:
flag=1
continue
if flag:
print line.strip()
# you can store the values of each option in this part