How to Split Strings with Delimiters Preserved Using Java Lookahead and Lookbehind?

Splitting Strings with Delimiters Preserved

When working with multiline strings delimited by a set of unique delimiters, it can be challenging to split the string while also preserving the delimiters themselves. The standard String.split method only separates the string based on the specified delimiter, discarding the delimiter characters.

Using Lookahead and Lookbehind

The Java Development Kit (JDK) provides a solution usinglookahead and lookbehind, which are features of regular expressions that allow you to match and capture characters without including them in the final result.

Here's an example using lookahead and lookbehind:

<code class="java">String input = "(Text1)(DelimiterA)(Text2)(DelimiterC)(Text3)(DelimiterB)(Text4)";

String[] splitWithDelimiter = input.split("((?<=;)|(?=;))");
System.out.println(Arrays.toString(splitWithDelimiter));</code>

The Pattern:

• (?<=;) matches an empty character directly after a semicolon (lookbehind).
• (?=;) matches an empty character directly before a semicolon (lookahead).
• ((?<=;)|(?=;)) combined matches an empty character either after or before a semicolon.

Output:

[Text1, ;, DelimiterA, ;, Text2, ;, DelimiterC, ;, Text3, ;, DelimiterB, ;, Text4]

This result preserves the delimiters along with the text, fulfilling the requirement of keeping the delimiters intact while splitting the string.

Improving Readability

Regular expressions can sometimes be difficult to read and understand. To enhance readability, you can create a variable with a descriptive name that represents the regular expression pattern. For example:

<code class="java">static final String WITH_DELIMITER = "((?<=%1$s)|(?=%1$s))";

public void someMethod() {
    final String[] splitResults = input.split(String.format(WITH_DELIMITER, ";"));
}</code>

By using placeholders and Java's String.format, you can make the regular expression more intuitive and easier to interpret.

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