java.lang.NumberFormatException: For input string: "4g3 (1)" [LUCENE-4405]

When trying to instantiate IndexWriter, I get this exception. I have no other clues on why this might happen.

Caused by: java.lang.NumberFormatException: For input string: "4g3 (1)"
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
at java.lang.Long.parseLong(Long.java:419)
at org.apache.lucene.index.SegmentInfos.generationFromSegmentsFileName(SegmentInfos.java:214)
at org.apache.lucene.index.SegmentInfos.getCurrentSegmentGeneration(SegmentInfos.java:149)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:626)
at org.apache.lucene.index.SegmentInfos$FindSegmentsFile.run(SegmentInfos.java:575)
at org.apache.lucene.index.SegmentInfos.read(SegmentInfos.java:346)
at org.apache.lucene.index.IndexWriter.<init>(IndexWriter.java:1115)

---
Migrated from [LUCENE-4405](https://issues.apache.org/jira/browse/LUCENE-4405) by Radhakrishna Bhat, updated Sep 11 2014
Environment:
```
Windows 7 64 bit JDK
```

Start with SegmentInfos.generationFromSegmentsFileName and the surrounding read path in SegmentInfos.java, using the provided IndexWriter stack trace as the entry point. Reproduce the failure on the stated Windows/JDK environment and determine why "4g3 (1)" is passed to Long.parseLong; done means the cause and expected handling are established, with a regression test if the project identifies a fixable behavior.

Written by the indexing model from the issue text.