regex - java regular expression find and replace

Question

I am trying to find environment variables in input and replace them with values.

The pattern of env variable is ${\.}

Pattern myPattern = Pattern.compile( "(${\.})" );
String line ="${env1}sojods${env2}${env3}";

How can I replace env1 with 1 and env2 with 2 and env3 with 3, so that after this I will have a new string 1sojods23?

Answer 1

Strings in Java are immutable, which makes this somewhat tricky if you are talking about an arbitrary number of things you need to find and replace.

Specifically you need to define your replacements in a Map, use a StringBuilder (before Java 9, less performant StringBuffer should have been used) and the appendReplacements() and appendTail() methods from Matcher. The final result will be stored in your StringBuilder (or StringBuffer).

Map<String, String> replacements = new HashMap<String, String>() {{
    put("${env1}", "1");
    put("${env2}", "2");
    put("${env3}", "3");
}};

String line ="${env1}sojods${env2}${env3}";
String rx = "(\$\{[^}]+\})";

StringBuilder sb = new StringBuilder(); //use StringBuffer before Java 9
Pattern p = Pattern.compile(rx);
Matcher m = p.matcher(line);

while (m.find())
{
    // Avoids throwing a NullPointerException in the case that you
    // Don't have a replacement defined in the map for the match
    String repString = replacements.get(m.group(1));
    if (repString != null)    
        m.appendReplacement(sb, repString);
}
m.appendTail(sb);

System.out.println(sb.toString());

Output:

1sojods23