How to check if string contains only letters, numbers and double backslash

Question

How can I write a method to check if string contains only numbers, letters, specific characters ( +_-/ ) and only double backslashes ( \\\\ ).

example:

a2b/\\ - false

d+sd\\\\ff - true

I tried this:

stringExample.matches("^[a-z-A-Z-0-9 +_-/]+");

...but it doesn't match a double backslash ( \\\\ ).

Answer 1

Since backslashes are generally used as the escape character, they have to be escapsed as well via a \\\\ to represent a single slash, so two consecutive backslashes would use \\\\\\\\ .

So for your expression to allow those characters and only sets of two backslashes, you could use :

stringExample.matches("^([\\w\\s+\\-/]|(\\\\\\\\))+$");

This can be explained as follows :

^             # Beginning of expression
(             # Beginning of group
[\\w\\s+\\-/] # Match any alphanumeric or underscore (\w), space (\s), or +, - or \
|             # OR
(\\\\\\\\)    # An explicit double-slash
)             # End of group
+             # Allow one or more instances of the previous group
$             # End of expression

Answer 2

\\ is a special character. It has functionality in regex (escaping). This means that you need to escape it in order to capture it as it's own character.

\\\\ would mean '\\' as a character

Answer 3

This might help:

private static void checkdoubleSlash(String a) {
        if(a.contains("\\\\"))
            System.out.println("True");
        else
            System.out.println("False");
    }

Answer 4

First of all, the pattern as you wrote it doesn't compile:

^[a-z-A-Z-0-9 +_-/]+

Because _-/ is interpreted as a range, but that's invalid. It's better to move the - to the end, to make it not be a range:

^[a-z-A-Z-0-9 +_/-]+

And also remove the duplicated -

^[a-zA-Z0-9 +_/-]+

Next, notice that [...] is for single characters. And you don't to allow single \\ , only double \\\\ , so that's a two-character pattern. To achieve this, the regular expression should be something like:

^([a-zA-Z0-9 +_/-]|double-backslash)+

I wrote "double-backslash there because this needs a bit of extra explanation. To write that, is not simply \\\\ , because in Java strings you need to escape backslashes, so \\\\\\\\ . And in regular expressions, you need to escape them again. So the final regular expression you need is:

^([a-zA-Z0-9 +_/-]|\\\\\\\\)+

With some unit tests:

public boolean testMatch(String s) {
    return s.matches("^([a-zA-Z0-9 +_/-]|\\\\\\\\)+");
}

@Test
public void should_match_string_with_double_backslash() {
    assertTrue(testMatch("d+sd\\\\ff"));
}

@Test
public void should_not_match_string_with_single_backslash() {
    assertFalse(testMatch("a2b/\\"));
}