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In text retrieval, full text search (also called free search text
The most common approach to full text search is to generate a complete index or concordance for all of the searchable documents. For each word (excepting stop words which are too common to be useful) an entry is made which lists the exact position of every occurrence of it within the database of documents. From such a list it is relatively simple to retrieve all the documents that match a query, without having to scan each document. Although for very small document collections full-text searching can be done by serial scanning, indexing is the preferred method for almost all full-text searching.

The precision vs. recall tradeoff
The deficiencies of free text searching have been addressed in two ways: By providing users with tools that enable them to express their search questions more precisely, and by developing new search algorithms that improve retrieval precision.

Improving the performance of full text searching

Keywords. Document creators (or trained indexers) are asked to supply a list of words that describe the subject of the text, including synonyms of words that describe this subject. Keywords improve recall, particularly if the keyword list includes a search word that is not in the document text.
Field-restricted search. Some search engines enable users to limit free text searches to a particular field within a stored data record, such as "Title" or "Author."
Boolean queries. Searches that use Boolean operators (for example, "encyclopedia" AND "online" NOT "Encarta") can dramatically increase the precision of a free text search. The AND operator says, in effect, "Do not retrieve any document unless it contains both of these terms." The NOT operator says, in effect, "Do not retrieve any document that contains this word." If the retrieval list retrieves too few documents, the OR operator can be used to increase recall; consider, for example, "encyclopedia" AND "online" OR "Internet" NOT "Encarta". This search will retrieve documents about online encyclopedias that use the term "Internet" instead of "online." This increase in precision is very commonly counter-productive since it usually comes with a dramatic loss of recall.
Phrase search. A phrase search matches only those documents that contain a specified phrase, such as "Wikipedia, the free encyclopedia."
Concordance search. A concordance search produces an alphabetical list of all principal words that occur in a text with their immediate context.
Proximity search. A phrase search matches only those documents that contain two or more words that are separated by a specified number of words; a search for "Wikipedia" WITHIN2 "free" would retrieve only those documents in which the words "Wikipedia" and "free" occur within two words of each other.
Regular expression. A regular expression employs a complex but powerful querying syntax that can be used to specify retrieval conditions with precision.
Wildcard search. A search that substitutes one or more characters in a search query for a wildcard character such as an asterisk. For example, in the search function in Microsoft Word, using the asterisk in the search query "s*n" will find "sin", "son", "sun", etc. in a text. Improved querying tools
Technological advances have greatly improved the performance of free text searching. For example, Google's PageRank algorithm gives more prominence to documents to which other Web pages have linked. This algorithm dramatically improves users' perception of search precision, a fact that explains its popularity among Internet users. See search engine for additional examples.

Text retrieval software