Regular expression to extract n-values from a string - regex

Question

Can regex extract the values embedded within a string, as identified by a variable template defined earlier within the same string? Or is this better handled in Java?
For example: "2012 Ferrari [F12] - Ostrich Leather interior [F12#OL] - Candy Red Metallic [F12#3]" The variable template is the first string encountered with square brackets, e.g. [F12], and the desired variables are found within subsequent instances of that template, e.g. 'OL' and '3'.

Answer 1

Since you are mentioning Java, I'll assume you are using the Java implementation, Pattern.
Java's Pattern supports so called back references, which can be used to match the same value a previous capturing group matched.
Unfortunately you cannot extract multiple values from a single capturing group, so you'll have to hardcode the number of templates you want to match, if you want to do this with a single pattern.
For one variable, it could look like this:
\[(.*?)\].*?\[\1#(.*?)\]
^^^^^ ^^^^^ variable
template ^^ back reference to whatever template matched
You can add more optional matches by wrapping them in optional non-capturing groups like this:
\[(.*?)\].*?\[\1#(.*?)\](?:.*?\[\1#(.*?)\])?(?:.*?\[\1#(.*?)\])?
^ optional group ^ another one
This would match up to three variables:
String s = "2012 Ferrari [F12] - Ostrich Leather interior [F12#OL] - Candy Red Metallic [F12#3]";
String pattern = "\\[(.*?)\\].*?\\[\\1#(.*?)\\](?:.*?\\[\\1#(.*?)\\])?(?:.*?\\[\\1#(.*?)\\])?";
Matcher matcher = Pattern.compile(pattern).matcher(s);
if (matcher.find()) {
for (int i = 1; i <= matcher.groupCount(); i++) {
System.out.println(matcher.group(i));
}
}
// prints F12, OL, 3, null
If you'll need to match any number of variables, however, you'll have to resort to extracting the template in a first pass and then embedding it in a second pattern:
// compile once and store in a static variable
Pattern templatePattern = Pattern.compile("\\[(.*?)\\]");
String s = "2012 Ferrari [F12] - Ostrich Leather interior [F12#OL] - Candy Red Metallic [F12#3]";
Matcher templateMatcher = templatePattern.matcher(s);
if (!templateMatcher.find()) {
return;
}
String template = templateMatcher.group(1);
Pattern variablePattern = Pattern.compile("\\[" + Pattern.quote(template) + "#(.*?)\\]");
Matcher variableMatcher = variablePattern.matcher(s);
while (variableMatcher.find()) {
System.out.println(variableMatcher.group(1));
}