grep -oh "[[:alpha:]]*th[[:alpha:]]*" 'filename'
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This is an updated and cross distribution safe answer
grep -oh "[[:alpha:]]*th[[:alpha:]]*" 'filename'
To summaries -oh outputs the regular expression matches to the file content (and not its filename), just like how you would expect regular expression to work in vim/etc... What word or regular expression you would be searching for then, is up to you! As long as you remain to POSIX and not perl syntax (refer below)
More from the manual for grep
-o Print each match, but only the match, not the entire line.
-h Never print filename headers (i.e. filenames) with output lines.
-w The expression is searched for as a word (as if surrounded by
`[[:<:]]' and `[[:>:]]';
The reason why the original answer does not work for everyone
Can grep show only words that match search pattern?
于是，就有了，查看weblogic GC日志的命令
在多个weblogic服务器日志上找FullGC
//查找FullGC grep 'Full' `head -n 10 weblogic.log |grep -oh 'gc_.*log'`
查找日志末5000行，打印文件名和错误的行
tail -n 50000 */weblogic*.log |grep -E "weblogic.*log|2937u38338"
-E选项，支持扩展的正则表达式，支持或“|”操作