CODEWITHSUNDEEP
javastring
user812142
user812142

Reputation: 173

Regular Expressions with double backslash in java

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?

Upvotes: 6

Views: 10982

Answers (3)

dpr
dpr

Reputation: 10964

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'

Upvotes: 9

m0skit0
m0skit0

Reputation: 25873

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 (e.g. \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.

Upvotes: 1

luk2302
luk2302

Reputation: 57124

$ 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.

Upvotes: 5

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