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Java regular expressions using replaceAll?

I have the following problem which states

"Replace all characters in a string with + symbol except instances of the given string in the method"

so for example if the string given was abc123efg and they want me to replace every character except every instance of "123" then it would become +++123+++.

I figured a regular expression is probably the best for this and I came up with this.

str.replaceAll("[^str]","+")


where str is a variable, but its not letting me use the method without putting it in quotations. If I just want to replace the variable string str how can I do that? I ran it with the string manually typed and it worked on the method, but can I just input a variable?

as of right now I believe its looking for the string "str" and not the variable string.

Here is the output its right for so many cases except for two :(

var.replaceAll("[^" + str + "]","+")

2 Answers
2

This is a bit trickier than you might initially think because you don't just need to match characters, but the absence of specific phrase - a negated character set is not enough. If the string is 123, you would need:

(?<=^|123)(?!123).*?(?=123|$)


https://regex101.com/r/EZWMqM/1/

That is - lookbehind for the start of the string or "123", make sure the current position is not followed by 123, then lazy-repeat any character until lookahead matches "123" or the end of the string. This will match all characters which are not in a "123" substring. Then, you need to replace each character with a +, after which you can use appendReplacement and a StringBuffer to create the result string:

+

appendReplacement

StringBuffer

String inputPhrase = "123";
String inputStr = "abc123efg123123hij";
StringBuffer resultString = new StringBuffer();
Pattern regex = Pattern.compile("(?<=^|" + inputPhrase + ")(?!" + inputPhrase + ").*?(?=" + inputPhrase + "|$)");
Matcher m = regex.matcher(inputStr);
while (m.find())
String replacement = m.group(0).replaceAll(".", "+");
m.appendReplacement(resultString, replacement);

m.appendTail(resultString);
System.out.println(resultString.toString());


Output:

+++123+++123123+++


Note that if the inputPhrase can contain character with a special meaning in a regular expression, you'll have to escape them first before concatenating into the pattern.

inputPhrase

So instead of coming up with a regular expression that matches the absence of a string. We might as well just match the selected phrase and append + the number of skipped characters.

+

StringBuilder sb = new StringBuilder();
Matcher m = Pattern.compile(Pattern.quote(str)).matcher(input);
while (m.find())
for (int i = 0; i < m.start(); i++) sb.append('+');
sb.append(str);

int remaining = input.length() - sb.length();
for (int i = 0; i < remaining; i++)
sb.append('+');



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