The letter tokenizer discards all non-letter characters from the input string and then generates a token at strings of contiguous letters.
Factory class: solr.LetterTokenizerFactory
Arguments: None
Example:
<fieldType name="text_en" class="solr.TextField" positionIncrementGap="100">
<analyzer>
<tokenizer class="solr.LetterTokenizerFactory"/>
</analyzer>
</fieldType>
Input: I haven't received mail by Nov12Sunday
Output: I, haven, t, received, mail, by, Nov, Sunday
All non-letter characters (' and 12) are discarded first, and then tokens are generated by considering strings of contiguous letters.