java - How to escape a square bracket for Pattern compilation?

Question

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].

Answer 1

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 - 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.