ppkarwasz commented on issue #4001:
URL:
https://github.com/apache/logging-log4j2/issues/4001#issuecomment-3951006227
Hi @snicoll,
Sorry for the delay: busy times at my day job.
The issue appears to be caused by the `filePattern` used in
spring-projects/spring-boot#47260:
```xml
<RollingFile name="File" fileName="${sys:LOG_FILE}"
filePattern="${sys:LOG4J2_ROLLINGPOLICY_FILE_NAME_PATTERN:-${sys:LOG_FILE}.%d{yyyy-MM-dd}.%i.gz}">
```
After substitution, this effectively becomes:
```
myapp.log.%d{yyyy-MM-dd.%i.gz}
```
This string matches the regex used to detect `%i`
(`.*%(?<ZEROPAD>0)?(?<PADDING>\d+)?i.*`), but it does **not** actually contain
a real `%i` rollover token: the `%i` is inside the date format. That mismatch
triggers the failure described in this issue.
### Expansion logic
This happens because `StrSubstitutor` does not support escaping `}` (see
apache/logging-log4j2#2679). Although it supports nested `${...}` expressions,
it treats the `}` in `%d{yyyy-MM-dd}` as the closing brace of the outer
substitution.
As a result, the default value is prematurely terminated and effectively
parsed as:
```
${sys:LOG_FILE}.%d{yyyy-MM-dd
```
which expands to:
```
myapp.log.%d{yyyy-MM-dd
```
The remaining `.%i.gz}` is then treated as a **literal** suffix and appended
unchanged.
### Workaround
Instead of using the `:-` default syntax, define the default via a dedicated
property:
```xml
<Properties>
<Property name="LOG4J2_ROLLINGPOLICY_FILE_NAME_PATTERN"
value="${sys:LOG_FILE}.%d{yyyy-MM-dd}.%i.gz"/>
</Properties>
<Appenders>
<RollingFile name="File"
fileName="${sys:LOG_FILE}"
filePattern="${sys:LOG4J2_ROLLINGPOLICY_FILE_NAME_PATTERN}">
```
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