How to split a regular expression into parts? - regex

Question

I need to break down a regular expression into its basic parts. For instance, given the regex [a-d]+[r-z]* I need to split it into [a-d]+ and [r-z]*. This is of course a very simple example, and regex syntax can get very complex...
Is there a (relatively) simple way to achieve this, or am I doomed to reverse-engineer a regex parser?
I need this to find out if a given string is a part of matching input for a given regular expression.

Answer 1

You can brute-force it this way:
import java.util.ArrayList;
import java.util.List;
import java.util.regex.Pattern;
import java.util.regex.PatternSyntaxException;
public class RegexSplitter {
private static boolean tryRegex(String regex) {
try {
Pattern.compile(regex);
return true;
} catch(PatternSyntaxException pse) {
return false;
}
}
public static void main(String args[]) {
String input = "[a-d]+[r-z]*";
List<String> results = new ArrayList<>();
int start = 0;
int end = 1;
boolean good = false;
while(end < input.length()) {
String part = input.substring(start, end);
if(!tryRegex(part)) {
if(good) {
good = false;
results.add(input.substring(start, end - 1));
start = end-1;
}
} else {
good = true;
}
++end;
}
if(tryRegex(input))
results.add(input.substring(start,end));
System.out.println(results);
}
}
// Output: [[a-d]+, [r-z]*]
It's hacky and heuristic, but it may work for your purposes.

Answer 2

Not sure if that's what you're asking, but there are several tools out there like RegexBuddy that you can use to analyze regexes.
Some languages like Python provide debug modes for regexes:
>>> import re
>>> re.compile(r"[a-d]+[r-z]*", re.DEBUG)
max_repeat 1 4294967295
in
range (97, 100)
max_repeat 0 4294967295
in
range (114, 122)