Unclosed character class while replacing string with regex

Question

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 ?

The \n in "\n" is a newline, not a backslash and n. So, it should be str = str.replaceFirst("\n[0-9]+\\s+:\\s*", ""). Do not use str.matches. Using g (global flag to match all occurrences) with .replaceFirst is not only invalid, it is not logical. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Dec 13, 2021 at 20:46
Thanks @WiktorStribiżew I don't have error now, but it didn't find something — Elikill58
– Elikill58, Commented Dec 13, 2021 at 20:48
See ideone.com/iMAFJS, what did you mean by "it didn't find something"? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Dec 13, 2021 at 20:50
@WiktorStribiżew I didn't change the string. But I found the issue, when I copy paste, I made a typo error. So yes, your regex works fine thanks ! Can you make an answer for this ? :) — Elikill58
– Elikill58, Commented Dec 13, 2021 at 20:54

Wiktor Stribiżew · Accepted Answer · 2021-12-13 20:58:44Z

2

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"

answered Dec 13, 2021 at 20:58

Wiktor Stribiżew

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