使用Java Regex删除字符串开头的给定字符序列的出现次数

Question

I have a string that begins with one or more occurrences of the sequence "Re:" . This "Re:" can be of any combinations, for ex. Re<any number of spaces>: , re: , re<any number of spaces>: , RE: , RE<any number of spaces>: , etc.

Sample sequence of string : Re: Re : Re : re : RE: This is a Re: sample string.
I want to define a java regular expression that will identify and strip off all occurrences of Re: , but only the ones at the beginning of the string and not the ones occurring within the string.
So the output should look like This is a Re: sample string.
Here is what I have tried:

String REGEX = "^(Re*\\p{Z}*:?|re*\\p{Z}*:?|\\p{Z}Re*\\p{Z}*:?)";
String INPUT = title;
String REPLACE = "";
Pattern p = Pattern.compile(REGEX);
Matcher m = p.matcher(INPUT);
while(m.find()){
  m.appendReplacement(sb,REPLACE);
}
m.appendTail(sb);

I am using p{Z} to match whitespaces(have found this somewhere in this forum, as Java regex does not identify \\s ).

The problem I am facing with this code is that the search stops at the first match, and escapes the while loop.

Answer 1

Try something like this replace statement:

yourString = yourString.replaceAll("(?i)^(\\s*re\\s*:\\s*)+", "");

Explanation of the regex:

(?i)  make it case insensitive
^     anchor to start of string
(     start a group (this is the "re:")
\\s*  any amount of optional whitespace
re    "re"
\\s*  optional whitespace
:     ":"
\\s*  optional whitespace
)     end the group (the "re:" string)
+     one or more times

Answer 2

in your regex:

String regex = "^(Re*\\p{Z}*:?|re*\\p{Z}*:?|\\p{Z}Re*\\p{Z}*:?)"

here is what it does:

see it live here

it matches strings like:

• \\p{Z}Reee\\p{Z: or
• R\\p{Z}}}

which make no sense for what you try to do:

you'd better use a regex like the following:

yourString.replaceAll("(?i)^(\\s*re\\s*:\\s*)+", "");

or to make @Doorknob happy, here's another way to achieve this, using a Matcher :

Pattern p = Pattern.compile("(?i)^(\\s*re\\s*:\\s*)+");
Matcher m = p.matcher(yourString);
if (m.find())
    yourString = m.replaceAll("");

(which is as the doc says the exact same thing as yourString.replaceAll() )

Look it up here

(I had the same regex as @Doorknob, but thanks to @jlordo for the replaceAll and @Doorknob for thinking about the (?i) case insensitivity part ;-) )