Some special characters are interpreted by Recoll in search strings to expand or specialize the search. Wildcards expand a root term in controlled ways. Anchor characters can restrict a search to succeed only if the match is found at or near the beginning of the document or one of its fields.
All words entered in Recoll search fields will be processed for wildcard expansion before the request is finally executed.
The wildcard characters are:
* which matches 0 or more characters.
? which matches a single character.
[] which allow defining sets of characters to be matched (ex: [abc] matches a single character which may be 'a' or 'b' or 'c', [0-9] matches any number.
You should be aware of a few things before using wildcards.
Using a wildcard character at the beginning of a word can make for a slow search because Recoll will have to scan the whole index term list to find the matches.
When working with a raw index (preserving character case and diacritics), the literal part of a wildcard expression will be matched exactly for case and diacritics.
Using a * at the end of a word can produce more matches than you would think, and strange search results. You can use the term explorer tool to check what completions exist for a given term. You can also see exactly what search was performed by clicking on the link at the top of the result list. In general, for natural language terms, stem expansion will produce better results than an ending * (stem expansion is turned off when any wildcard character appears in the term).
Two characters are used to specify that a search hit should occur at the beginning or at the end of the text. ^ at the beginning of a term or phrase constrains the search to happen at the start, $ at the end force it to happen at the end.
As this function is implemented as a phrase search it is possible to specify a maximum distance at which the hit should occur, either through the controls of the advanced search panel, or using the query language, for example, as in:
"^someterm"o10which would force someterm to be found within 10 terms of the start of the text. This can be combined with a field search as in somefield:"^someterm"o10 or somefield:someterm$.
This feature can also be used with an actual phrase search, but in this case, the distance applies to the whole phrase and anchor, so that, for example, bla bla my unexpected term at the beginning of the text would be a match for "^my term"o5.
Anchored searches can be very useful for searches inside somewhat structured documents like scientific articles, in case explicit metadata has not been supplied (a most frequent case), for example for looking for matches inside the abstract or the list of authors (which occur at the top of the document).