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I have comma separated list of regular expressions:

.{8},[0-9],[^0-9A-Za-z ],[A-Z],[a-z]

I have done a split on the comma. Now I'm trying to match this regex against a generated password. The problem is that Pattern.compile does not like square brackets that is not escaped.

Can some please give me a simple function that takes a string like so: [0-9] and returns the escaped string \[0-9\].

informatik01
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Afamee
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4 Answers4

51

For some reason, the above answer didn't work for me. For those like me who come after, here is what I found.

I was expecting a single backslash to escape the bracket, however, you must use two if you have the pattern stored in a string. The first backslash escapes the second one into the string, so that what regex sees is \]. Since regex just sees one backslash, it uses it to escape the square bracket.

\\]

In regex, that will match a single closing square bracket.

If you're trying to match a newline, for example though, you'd only use a single backslash. You're using the string escape pattern to insert a newline character into the string. Regex doesn't see \n - it sees the newline character, and matches that. You need two backslashes because it's not a string escape sequence, it's a regex escape sequence.

Cullub
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    When thinking about it I came up why this is like that: The regex is a String and whatever processes this regex will look for a single backslah as an escape character. However as the regex is passed as a String you have to escape the backslah as well in order to get it into a String properly and that's the readon why you need two backslashes – Raven Mar 27 '16 at 18:51
29

You can use Pattern.quote(String).

From the docs:

public static String quote​(String s)

Returns a literal pattern String for the specified String.

This method produces a String that can be used to create a Pattern that would match the string s as if it were a literal pattern.

Metacharacters or escape sequences in the input sequence will be given no special meaning.

Laurence Gonsalves
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  • What value do you put in for `String`? `Pattern.quote("\[0-9\]")`? – Danny Bullis Apr 10 '18 at 22:16
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    @DannyBullis From the question "a simple function that takes a string like so: `[0-9]` and returns the escaped string `\[0-9\]`". So you'd give this `"[0-9]"`, and it will return something equivalent to `"\[0-9\]"`. (It actually uses `\Q` and `\E`, but the end result has the same effect when given to `Pattern.compile`.) – Laurence Gonsalves Apr 10 '18 at 22:28
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    awesome. Thanks for the quick help, even 9 years later :) – Danny Bullis Apr 11 '18 at 00:45
15

You can use the \Q and \E special characters...anything between \Q and \E is automatically escaped.

\Q[0-9]\E
Dan Breen
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  • Sounds a bit perlish if you ask me, have you tried it in java (I havn't, that's why I ask). – Fredrik Jul 18 '09 at 08:30
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    It's valid in Java too: http://java.sun.com/javase/6/docs/api/java/util/regex/Pattern.html (ctrl-F for "\Q") – Tyler Jul 18 '09 at 08:48
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    In Java string literal format it would be "\\Q[0-9]\\E" or "\\Q" + regex + "\\E". But the quote() method does that for you, plus it deals correctly with strings that already have \E in them. – Alan Moore Jul 19 '09 at 04:55
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Pattern.compile() likes square brackets just fine. If you take the string 

".{8},[0-9],[^0-9A-Za-z ],[A-Z],[a-z]"

and split it on commas, you end up with five perfectly valid regexes: the first one matches eight non-line-separator characters, the second matches an ASCII digit, and so on. Unless you really want to match strings like ".{8}" and "[0-9]", I don't see why you would need to escape anything.

Alan Moore
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