Question

Say I have a file, that contains some text. There are substrings like “substr1”, “substr2”, “substr3” etc. in it. I need to replace all of those substrings with some other text, like “repl1”, “repl2”, “repl3”. In Python, I would create a dictionary like this:

{
 "substr1": "repl1",
 "substr2": "repl2",
 "substr3": "repl3"
}

and create the pattern joining the keys with ‘|’, then replace with re.sub function.
Is there a similar simple way to do this in Java?

Answer 1

This is how your Python-suggestion translates to Java:

Map<String, String> replacements = new HashMap<String, String>() {{
    put("substr1", "repl1");
    put("substr2", "repl2");
    put("substr3", "repl3");
}};

String input = "lorem substr1 ipsum substr2 dolor substr3 amet";

// create the pattern joining the keys with '|'
String regexp = "substr1|substr2|substr3";

StringBuffer sb = new StringBuffer();
Pattern p = Pattern.compile(regexp);
Matcher m = p.matcher(input);

while (m.find())
    m.appendReplacement(sb, replacements.get(m.group()));
m.appendTail(sb);


System.out.println(sb.toString());   // lorem repl1 ipsum repl2 dolor repl3 amet

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This approach does a simultanious (i.e. “at once”) replacement. I.e., if you happened to have

"a" -> "b"
"b" -> "c"

then this approach would give "a b" -> "b c" as opposed to the answers suggesting you should chain several calls to replace or replaceAll which would give "c c".

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(If you generalize this approach to create the regexp programatically, make sure you Pattern.quote each individual search word and Matcher.quoteReplacement each replacement word.)