Question

I have a string, for ex:

There exists a word *random*.

random will be a random word.
How can I use a regular expression to replace every character of random with * and have this result:

There exists a word ********.

So the * replaces every character, in this case 6 characters.
Notice that I am after to replace only the word random, not the surroundings *.
So far I have:

str.replaceAll("(\\*)[^.]*(\\*)", "\\*");

But it replaces *random* with *, instead of the desired ******** (total of 8).
Any help, really appreciated…

Answer 1

If you have just a single word like that: –

As far as current example is concerned, if you are having just a single word like that, then you can save yourself from regex, by using some String class methods: –

String str = "There exists a word *random*.";

int index1 = str.indexOf("*");
int index2 = str.indexOf("*", index1 + 1);

int length = index2 - index1 - 1;   // Get length of `random`

StringBuilder builder = new StringBuilder();

// Append part till start of "random"
builder.append(str.substring(0, index1 + 1));

// Append * of length "random".length()
for (int i = 0; i < length; i++) {
    builder.append("*");
}

// Append part after "random"
builder.append(str.substring(index2));

str = builder.toString();

---

If you can have multiple words like that: –

For that, here’s a regex solution (This is where it starts getting a little complex): –

String str = "There exists a word *random*.";
str = str.replaceAll("(?<! ).(?!([^*]*[*][^*]*[*])*[^*]*$)", "*");
System.out.println(str);

The above pattern replaces all the characters that is not followed by string containing even numbers of * till the end, with a *.

Whichever is appropriate for you, you can use.

I’ll add an explanation of the above regex: –

(?<! )       // Not preceded by a space - To avoid replacing first `*`
.            // Match any character
(?!          // Not Followed by (Following pattern matches any string containing even number of stars. Hence negative look-ahead
    [^*]*    // 0 or more Non-Star character
    [*]      // A single `star`
    [^*]*    // 0 or more Non-star character
    [*]      // A single `star`
)*           // 0 or more repetition of the previous pattern.
[^*]*$       // 0 or more non-star character till the end.     

Now the above pattern will match only those words, which are inside a pair of stars. Provided you don’t have any unbalanced stars.