Fun with Regular Expressions: ANT-style Variable Replacing in Strings

i recently felt the need to write a piece of code that resolves ant-style variables in a string. suppose you have a property file similar to this one:

propertya=somevalue
propertyb=${propertya}.someothervalue
listofthings=${propertya}, ${propertyb}, constantvalue

let's further assume you want to read property listofthings , and resolve the variables. isn't that a perfect job for regular expressions?

using regex tester , i came up with the following regular expression to find occurrence of ${ variable }:

\$\{(.+?)\}

using java.util.regex.pattern and java.util.regex.matcher to find all occurrences is rather trivial:

pattern re = pattern.compile("\\$\\{(.+?)\\}");
matcher m = re.matcher(sourcestring);
while (m.find()) {
  string variable = m.group(1);
  system.out.println(variable);
}

replacing the variables with their concrete values is not so trivial. you might be tempted to use string.substring():

value = sourcestring.substring(0, m.start()) + replacement + (sourcestring.substring(m.end()));

but this will modify the source string, basically throwing the matcher out of the curve.

looking at the matcher api, i found java.util.regex.matcher.appendreplacement(stringbuffer, string) and java.util.regex.matcher.appendtail(stringbuffer). these two little gems to the trick:

private string resolve(string sourcestring, properties props) {
  pattern re = pattern.compile("\\$\\{(.+?)\\}");
  matcher m = re.matcher(sourcestring);
  stringbuffer result = new stringbuffer();
  while (m.find()) {
    string variable = m.group(1);
    string resolved = resolve(props.getproperty(variable), props);
    m.appendreplacement(result, resolved);
  }
  m.appendtail(result);
  return result.tostring();
}

regular expressions do save the day!

from http://www.peterfriese.de