Lexical parsing converts text into tokens for further analysis. Tokens are generated by evaluating lexemes, which can be split by grammar rules. Validation is done separately. Lexical analysis is useful but often needs to be combined with other methods for meaningful results.
Lexical parsing is the process of taking a string of characters – or, more simply, text – and converting them into meaningful groups called tokens. This methodology has uses in a wide variety of applications, from interpreting computer languages to analyzing books. Lexical analysis is not synonymous with analysis; rather, it is the first step in the total analysis process and creates raw material for later use.
Token building blocks, also called lexemes, can be generated in many ways, depending on the grammar required for lexical parsing. A common example of this is breaking sentences down by words; this is often done by splitting sentences around spaces. Any continuous string of characters generated without spaces is a lexeme. Text strings can be split over one or more character types, creating multiple versions of lexemes with varying complexity. Tokens are generated after each lexeme has been evaluated and matched to the corresponding value; by definition, tokens refer to this matching, not just the lexeme.
Lexical parsing somewhat counter-intuitively strips a text string of its context. Its purpose is only to generate building blocks for further study, not to determine whether those pieces are valid or invalid. In the case of computer language interpretation, validation is done by parsing the syntax and text validation can be done in terms of context or content. If an input string is completely split into appropriate lexemes and each of these lexemes has an appropriate value, the parse is considered successful.
Without context or the ability to perform validation, lexical analysis cannot be reliably used to find errors in input. A lexical grammar might have error values assigned to specific lexemes, and such an analysis can also detect illegal or malformed tokens. While finding an illegal or malformed token signals invalid input, it has no bearing on the validity of the other tokens and is therefore not strictly a type of validation.
While lexical analysis is an integral part of many algorithms, it often needs to be used in conjunction with other methodologies to create meaningful results. For example, splitting a text string into words to determine frequencies makes use of lexeme creation, but lexeme creation alone cannot track the number of times a particular lexeme appears in input. Lexical analysis might be useful on its own if the lexemes themselves are noteworthy, but large amounts of input might make parsing the raw lexemes difficult due to the volume of data.
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