Question

I’m writing code that’s supposed to remove actual line breaks from a block of text and replace them with the String “\n”. Then, when the String is read at another time, it should replace the line breaks (in other words, search for all “\n” and insert \n. However, while the first conversion works fine, it’s not doing the latter. It seems as though the second replace is doing nothing. Why?

The replace:

theString.replaceAll(Constants.LINE_BREAK, Constants.LINE_BREAK_DB_REPLACEMENT);

The re-replace:

theString.replaceAll(Constants.LINE_BREAK_DB_REPLACEMENT, Constants.LINE_BREAK);

The constants:

public static final String LINE_BREAK = "\n";
public static final String LINE_BREAK_DB_REPLACEMENT = "\\\\n";

Answer 1

In String.replaceAll(regex, replacement), both the regex string and replacement string treat backslash as an escape character:

• regex represents a regular expression, which escapes a backslash as \\
• replacement is a replacement string, which also escapes backslashes:

Note that backslashes (\) and dollar signs ($) in the replacement string may cause the results to be different than if it were being treated as a literal replacement string; see Matcher.replaceAll.

This means backslashes must be escaped in both parameters. Further, string constants also use backslash as an escape character, so backslashes in string constants passed to the method must be double-escaped (see also this question).

This works fine for me:

// Replace newline with "\n"
theString.replaceAll("\\n", "\\\\n");

// Replace "\n" with newline
theString.replaceAll("\\\\n","\n");

You can also use the Matcher.quoteReplacement() method to treat the replacement string as a literal:

// Replace newline with "\n"
theString.replaceAll("\\n", Matcher.quoteReplacement("\\n"));
// Replace "\n" with newline
theString.replaceAll("\\\\n",Matcher.quoteReplacement("\n"));