问题
Let's say I have this text input.
tes{}tR{R{abc}aD{mnoR{xyz}}}
I want to extract the ff output:
R{abc}
R{xyz}
D{mnoR{xyz}}
R{R{abc}aD{mnoR{xyz}}}
Currently, I can only extract what's inside the {}groups using balanced group approach as found in msdn. Here's the pattern:
^[^{}]*(((?'Open'{)[^{}]*)+((?'Target-Open'})[^{}]*)+)*(?(Open)(?!))$
Does anyone know how to include the R{} and D{} in the output?
回答1:
I think that a different approach is required here. Once you match the first larger group R{R{abc}aD{mnoR{xyz}}} (see my comment about the possible typo), you won't be able to get the subgroups inside as the regex doesn't allow you to capture the individual R{ ... } groups.
So, there had to be some way to capture and not consume and the obvious way to do that was to use a positive lookahead. From there, you can put the expression you used, albeit with some changes to adapt to the new change in focus, and I came up with:
(?=([A-Z](?:(?:(?'O'{)[^{}]*)+(?:(?'-O'})[^{}]*?)+)+(?(O)(?!))))
[I also renamed the 'Open' to 'O' and removed the named capture for the close brace to make it shorter and avoid noises in the matches]
On regexhero.net (the only free .NET regex tester I know so far), I got the following capture groups:
1: R{R{abc}aD{mnoR{xyz}}}
1: R{abc}
1: D{mnoR{xyz}}
1: R{xyz}
Breakdown of regex:
(?= # Opening positive lookahead
([A-Z] # Opening capture group and any uppercase letter (to match R & D)
(?: # First non-capture group opening
(?: # Second non-capture group opening
(?'O'{) # Get the named opening brace
[^{}]* # Any non-brace
)+ # Close of second non-capture group and repeat over as many times as necessary
(?: # Third non-capture group opening
(?'-O'}) # Removal of named opening brace when encountered
[^{}]*? # Any other non-brace characters in case there are more nested braces
)+ # Close of third non-capture group and repeat over as many times as necessary
)+ # Close of first non-capture group and repeat as many times as necessary for multiple side by side nested braces
(?(O)(?!)) # Condition to prevent unbalanced braces
) # Close capture group
) # Close positive lookahead
The following will not work in C#
I actually wanted to try out how it should be working out on the PCRE engine, since there was the option to have recursive regex and I think it was easier since I'm more familiar with it and which yielded a shorter regex :)
(?=([A-Z]{(?:[^{}]|(?1))+}))
regex101 demo
(?= # Opening positive lookahead
([A-Z] # Opening capture group and any uppercase letter (to match R & D)
{ # Opening brace
(?: # Opening non-capture group
[^{}] # Matches non braces
| # OR
(?1) # Recurse first capture group
)+ # Close non-capture group and repeat as many times as necessary
} # Closing brace
) # Close of capture group
) # Close of positive lookahead
回答2:
I'm not sure a single regex would be able to suit your needs: these nested substrings always mess it up.
One solution could be the following algorithm (written in Java, but I guess the translation to C# won't be that hard):
/**
* Finds all matches (i.e. including sub/nested matches) of the regex in the input string.
*
* @param input
* The input string.
* @param regex
* The regex pattern. It has to target the most nested substrings. For example, given the following input string
* <code>A{01B{23}45C{67}89}</code>, if you want to catch every <code>X{*}</code> substrings (where <code>X</code> is a capital letter),
* you have to use <code>[A-Z][{][^{]+?[}]</code> or <code>[A-Z][{][^{}]+[}]</code> instead of <code>[A-Z][{].+?[}]</code>.
* @param format
* The format must follow the <a href= "http://docs.oracle.com/javase/7/docs/api/java/util/Formatter.html#syntax" >format string
* syntax</a>. It will be given one single integer as argument, so it has to contain (and to contain only) a <code>%d</code> flag. The
* format must not be foundable anywhere in the input string. If <code>null</code>, <code>ééé%dèèè</code> will be used.
* @return The list of all the matches of the regex in the input string.
*/
public static List<String> findAllMatches(String input, String regex, String format) {
if (format == null) {
format = "ééé%dèèè";
}
int counter = 0;
Map<String, String> matches = new LinkedHashMap<String, String>();
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(input);
// if a substring has been found
while (matcher.find()) {
// create a unique replacement string using the counter
String replace = String.format(format, counter++);
// store the relation "replacement string --> initial substring" in a queue
matches.put(replace, matcher.group());
String end = input.substring(matcher.end(), input.length());
String start = input.substring(0, matcher.start());
// replace the found substring by the created unique replacement string
input = start + replace + end;
// reiterate on the new input string (faking the original matcher.find() implementation)
matcher = pattern.matcher(input);
}
List<Entry<String, String>> entries = new LinkedList<Entry<String, String>>(matches.entrySet());
// for each relation "replacement string --> initial substring" of the queue
for (int i = 0; i < entries.size(); i++) {
Entry<String, String> current = entries.get(i);
// for each relation that could have been found before the current one (i.e. more nested)
for (int j = 0; j < i; j++) {
Entry<String, String> previous = entries.get(j);
// if the current initial substring contains the previous replacement string
if (current.getValue().contains(previous.getKey())) {
// replace the previous replacement string by the previous initial substring in the current initial substring
current.setValue(current.getValue().replace(previous.getKey(), previous.getValue()));
}
}
}
return new LinkedList<String>(matches.values());
}
Thus, in your case:
String input = "tes{}tR{R{abc}aD{mnoR{xyz}}}";
String regex = "[A-Z][{][^{}]+[}]";
findAllMatches(input, regex, null);
Returns:
R{abc}
R{xyz}
D{mnoR{xyz}}
R{R{abc}aD{mnoR{xyz}}}
回答3:
Balancing groups in .Net regular expressions give you control over exactly what to capture, and the .Net regex engine keeps a full history of all captures of the group (unlike most other flavors that only capture the last occurrence of each group).
The MSDN example is a little too complicated. A simpler approach for matching nestes structures would be:
(?>
(?<O>)\p{Lu}\{ # Push to the O stack, and match an upper-case letter and {
| # OR
\}(?<-O>) # Match } and pop from the stack
| # OR
\p{Ll} # Match a lower-case letter
)+
(?(O)(?!)) # Make sure the stack is empty
or in a single line:
(?>(?<O>)\p{Lu}\{|\}(?<-O>)|\p{Ll})+(?(O)(?!))
Working example on Regex Storm
In your example it also matches the "tes" at the start of the string, but don't worry about that, we're not done.
With a small correction we can also capture the occurrences between the R{...} pairs:
(?>(?<O>)\p{Lu}\{|\}(?<Target-O>)|\p{Ll})+(?(O)(?!))
Each Match will have a Group called "Target", and each such Group will have a Capture for each occurrences - you only care about these captures.
Working example on Regex Storm - Click on Table tab and examine the 4 captures of ${Target}
See also:
- What are regular expression Balancing Groups?
来源：https://stackoverflow.com/questions/19027034/how-to-make-balancing-group-capturing