Get group names in java regex

Question

I'm trying to receive both a pattern & a string and return a map of group name -> matched result.

Example:

(?<user>.*) 

I would like to return for a map containing "user" as a key and whatever it matches as its value.

the problem is that I can't seem to get the group name from the Java regex api. I can only get the matched values by name or by index. I don't have the list of group names and neither Pattern nor Matcher seem to expose this information. I have checked its source and it seems as if the information is there - it's just not exposed to the user.

I tried both Java's java.util.regex and jregex. (and don't really care if someone suggested any other library that is good, supported & high in terms performance that supports this feature).

Answer 1

There is no API in Java to obtain the names of the named capturing groups. I think this is a missing feature.

The easy way out is to pick out candidate named capturing groups from the pattern, then try to access the named group from the match. In other words, you don't know the exact names of the named capturing groups, until you plug in a string that matches the whole pattern.

The Pattern to capture the names of the named capturing group is \(\?<([a-zA-Z][a-zA-Z0-9]*)> (derived based on Pattern class documentation).

(The hard way is to implement a parser for regex and get the names of the capturing groups).

A sample implementation:

import java.util.Scanner; import java.util.Set; import java.util.TreeSet; import java.util.Iterator; import java.util.regex.Pattern; import java.util.regex.Matcher; import java.util.regex.MatchResult;  class RegexTester {      public static void main(String args[]) {         Scanner scanner = new Scanner(System.in);          String regex = scanner.nextLine();         StringBuilder input = new StringBuilder();         while (scanner.hasNextLine()) {             input.append(scanner.nextLine()).append('\n');         }          Set<String> namedGroups = getNamedGroupCandidates(regex);          Pattern p = Pattern.compile(regex);         Matcher m = p.matcher(input);         int groupCount = m.groupCount();          int matchCount = 0;          if (m.find()) {             // Remove invalid groups             Iterator<String> i = namedGroups.iterator();             while (i.hasNext()) {                 try {                     m.group(i.next());                 } catch (IllegalArgumentException e) {                     i.remove();                 }             }              matchCount += 1;             System.out.println("Match " + matchCount + ":");             System.out.println("=" + m.group() + "=");             System.out.println();             printMatches(m, namedGroups);              while (m.find()) {                 matchCount += 1;                 System.out.println("Match " + matchCount + ":");                 System.out.println("=" + m.group() + "=");                 System.out.println();                 printMatches(m, namedGroups);             }         }     }      private static void printMatches(Matcher matcher, Set<String> namedGroups) {         for (String name: namedGroups) {             String matchedString = matcher.group(name);             if (matchedString != null) {                 System.out.println(name + "=" + matchedString + "=");             } else {                 System.out.println(name + "_");             }         }          System.out.println();          for (int i = 1; i < matcher.groupCount(); i++) {             String matchedString = matcher.group(i);             if (matchedString != null) {                 System.out.println(i + "=" + matchedString + "=");             } else {                 System.out.println(i + "_");             }         }          System.out.println();     }      private static Set<String> getNamedGroupCandidates(String regex) {         Set<String> namedGroups = new TreeSet<String>();          Matcher m = Pattern.compile("\\(\\?<([a-zA-Z][a-zA-Z0-9]*)>").matcher(regex);              while (m.find()) {                 namedGroups.add(m.group(1));             }              return namedGroups;         }     } } 

There is a caveat to this implementation, though. It currently doesn't work with regex in Pattern.COMMENTS mode.