Regular Expressions with double backslash in java

Question

I want to understand the concept of regular expression in below code:

private static final String SQL_INSERT = "INSERT INTO ${table}(${keys}) 
VALUES(${values})";
private static final String TABLE_REGEX = "\\$\\{table\\}";
.
.
.
String query = SQL_INSERT.replaceFirst(TABLE_REGEX, "tableName");

The above code is working fine but i would like to understand how. As per my knowledge $ and { symbols should be escaped in java string using backslash but in above string there is no backslash and if I try to add it, it shows error: invalid escape sequence.

Also why the TABLE_REGEX = "\\\\$\\\\{table\\\\}"; contains double backslash?

Answer 1

$ or "$" is the dollar sign / a string containg it.
\\$ is an escaped dollar sign, normally found in raw regex if you want to match the char $ instead of the end of the line
"\\\\$" is a String containing an escaped \\ followed by a normale $ . Since you are not writing a raw regex, but the regex is inside a Java String you need to escape the \\ so that when the regex interpreter comes along it just sees a normal \\ which it then treats as escaping the following $ .

"\\$" is not valid because from a normal String point of view a $ is nothing special and does not need to / must not be escaped.

Answer 2

The $ and { don't need to be escaped in Java string literals in general but in regular expressions they need to be escaped as they have special meaning in regular expressions. The $ matches the end of a line and { is used for matching characters a certain amount of times. To match any of the regular expression special characters themselves these characters need to be escaped. For example A{5} matches AAAAA but A\\{5 matches A{5 .

To escape something in a regular expression string you use the \\ . But the backslash in string literals itself needs escaping which is done by another \\ . That is the String literal "\\\\{" actually corresponds to the string "\\{" .

This is why in regular expression string literals you will often encounter multiple backslashes. You might also want to take a look at Pattern.quote(String s) which takes a string and properly escapes all special characters (wrt. Java regular expressions).

Essentially instead of

private static final String TABLE_REGEX = "\\$\\{table\\}";

you could write

private static final String TABLE_REGEX = Pattern.quote("${table}");

In your example SQL_INSERT.replaceFirst(TABLE_REGEX, "tableName"); matches the first occurrence of ${table} in SQL_INSERT and replaces this occurrence with tableName :

String sql = "INSERT INTO ${table}(${keys}) VALUES(${values})".replaceFirst("\\$\\{table\\}", "tableName");
boolean test = sql.equals("INSERT INTO tableName(${keys}) VALUES(${values})");
System.out.println(test); // will print 'true'

Answer 3

i would like to understand how.

It is replacing the first match of the regex "\\\\$\\\\{table\\\\}" in the original string "INSERT INTO ${table}(${keys}) VALUES(${values})" with "tableName" .

$ and { symbols should be escaped in java string using backslash but in above string there is no backslash and if I try to add it, it shows error: invalid escape sequence.

No, ${} are not escaped in a Java string, why would they?

Also why the TABLE_REGEX = "\\$\\{table\\}"; contains double backslash?

In Java escaping is done by double backslash because single backslash indicates special character (eg \\n , \\t ). It is escaping ${} symbols because these symbols have a special meaning in a regex, so escaping them tells the Java regex engine to treat them literally as those symbols and not their special meaning.