Java – regular expressions and escaped and non escaped separators

Problems related to this

I have a string

a\;b\\;c;d

Looks like in Java

String s = "a\\;b\\\\;c;d"

I need to split it with a semicolon according to the following rules:

>If a semicolon is preceded by a backslash, it should not be considered a separator (between a and b). > If the backslash itself is escaped and therefore not escaped as a semicolon, the semicolon should be a separator (between B and C)

Therefore, if there are zero or even backslashes before it, the semicolon should be regarded as a separator

For example, above, I want to get the following string (double backslash of java compiler):

a\;b\\
c
d

Solution

You can use regular expressions

(?:\\.|[^;\\]++)*

Match all text between non escaped semicolons:

List<String> matchList = new ArrayList<String>();
try {
    Pattern regex = Pattern.compile("(?:\\\\.|[^;\\\\]++)*");
    Matcher regexMatcher = regex.matcher(subjectString);
    while (regexMatcher.find()) {
        matchList.add(regexMatcher.group());
    }

explain:

(?:        # Match either...
 \\.       # any escaped character
|          # or...
 [^;\\]++  # any character(s) except semicolon or backslash; possessive match
)*         # Repeat any number of times.

Because of nested quantifiers, possessive matching () is very important to avoid catastrophic backtracking