Unclosed character class while replacing string with regex

I want to remove a string at the beginning of another.

Example:

Begin	Removed	Final
“n123 : other”	Yes	other
“123 : other”	No	Error
“n4 : smth”	Yes	smth
“123 : a”	No	Error

Thanks to Regexr, I made this regex:

/[\][n](?:[0-9]*)[ ][:]/g

I tried to use it in Java. With string.matches() I don’t have any error. But, with string.replaceFirst, I have this error :

java.util.regex.PatternSyntaxException: Unclosed character class near index 24
/[][n](?:[0-9]*)[ ][:]/g
                        ^

What I tried else ?

• Remove / at begin and /g at end.
• Add (pattern) (such as visible in regex here)
• Multiple others quick way, but I the error is always at the last char of my regex

My full code:

String str = "n512 : something"; // I init my string
if(str.matches("/[\][n](?:[0-9]*)[ ][:]/g"))
   str = str.replaceFirst("/[\][n](?:[0-9]*)[ ][:]/g", "");

How can I fix it ? How can I solve my regex pattern ?

Answer

You can use

.replaceFirst("n[0-9]+\s+:\s*", "")

The regex matches

• n – a newline char (you have a newline in the string, not a backslash and n)
• [0-9]+ – one or more digits
• s+ – one or more whitespaces
• : – a colon
• s* – zero or more whitespaces

See the Java demo:

List<String> strs = Arrays.asList("n123 : other", "123 : other", "n4 : smth", "123 : a");
for (String str : strs)
    System.out.println(""" + str.replaceFirst("n[0-9]+\s+:\s*", "") + """);

Output:

"other"
"123 : other"
"smth"
"123 : a"