In order to process XML documents, users typically need a special library: While the techniques were used for an XML parser, most of them can be applied to parsers of other formats or even unrelated software e. The parser invokes the callbacks according to the data in the document.
They are published here in case others find them useful, but I provide no warranty for their accuracy, completeness or whether or not they are up-to-date. The contents of this page have dubious copyright status, as great portions of some of my revision notes are verbatim from the lecture slides, what the lecturer wrote on the board, or what they said.
Additionally, lots of the images have been captured from the lecture slides. Languages are a potentially infinite set of strings sometimes called sentences, which are a sequence of symbols from a given alphabet.
The tokens of each sentence are ordered according to some structure. A grammar is a set of rules that describe a language. Grammars assign structure to a sentence. An automaton is an algorithm that can recognise accept all sentences of a language and reject those which do not belong to it.
Lexical and syntactical analysis can be simplified to a machine that takes in some program code, and then returns syntax errors, parse trees and data structures.
We can think of the process of description transformation, where we take some source description, apply a transformation technique and end up with a target description - this is inference mapping between two equivalent languages, where the destination is a machine executable.
Compilers are an important part of computer science, even if you never write a compiler, as the concepts are used regularly, in interpreters, intelligent editors, source code debugging, natural language processing e. Throughout the years, progress has been made in the field of programming to bring higher and higher levels of abstraction, moving from machine code, to assembler, to high level langauges and now to object orrientation, reusable languages and virtual machines.
The front-end of a compiler only analyses the program, it does not produce code. From source code, lexical analysis produces tokens, the words in a language, which are then parsed to produce a syntax tree, which checks that tokens conform with the rules of a language.
Semantic analysis is then performed on the syntax tree to produce an annotated tree. In addition to this, a literal table, which contains information on the strings and constants used in the program, and a symbol table, which stores information on the identifiers occuring in the program e.
An error handler also exists to catch any errors generated by any stages of the program e. The syntax tree forms an intermediate representation of the code structure, and has links to the symbol table. From the annotated tree, intermediate code generation produces intermediate code e.
Optimisation is then applied to the target code. The target code could be assembly and then passed to an assembler, or it could be direct to machine code. We can consider the front-end as a two stage process, lexical analysis and syntactic analysis. Lexical Analysis Lexical analysis is the extraction of individual words or lexemes from an input stream of symbols and passing corresponding tokens back to the parser.
If we consider a statement in a programming language, we need to be able to recognise the small syntactic units tokens and pass this information to the parser. We need to also store the various attributes in the symbol or literal tables for later use, e.
Other roles of the lexical analyser include the removal of whitespace and comments and handling compiler directives i. The tokenisation process takes input and then passes this input through a keyword recogniser, an identifier recogniser, a numeric constant recogniser and a string constant recogniser, each being put in to their own output based on disambiguating rules.
These rules may include "reserved words", which can not be used as identifiers common examples include begin, end, void, etcthus if a string can be either a keyword or an identifier, it is taken to be a keyword.
Another common rule is that of maximal munch, where if a string can be interpreted as a single token or a sequence of tokens, the former interpretation is generally assumed.
For revision of or lecture notes. The lexical analysis process starts with a definition of what it means to be a token in the language with regular expressions or grammars, then this is translated to an abstract computational model for recognising tokens a non-deterministic finite state automatonwhich is then translated to an implementable model for recognising the defined tokens a deterministic finite state automaton to which optimisations can be made a minimised DFA.
However, not all syntactically valid sentences are meaningful, further semantic analysis has to be applied for this.
For syntactic analysis, context-free grammars and the associated parsing techniques are powerful enough to be used - this overall process is called parsing. In syntactic analysis, parse trees are used to show the structure of the sentence, but they often contain redundant information due to implicit definitions e.
Trees are recursive structures, which complement CFGs nicely, as these are also recursive unlike regular expressions. There are many techniques for parsing algorithms vs FSA-centred lexical analysisand the two main classes of algorithm are top-down and bottom-up parsing.
For information about see MCS. BNF uses three classes of symbols: As derivations are ambiguous, a more abstract structure is needed. Parse trees generalise derivations and provide structural information needed by the later stages of compilation. Parse Trees Parse trees over a grammar G is a labelled tree with a root node labelled with the start symbol Sand then internal nodes labelled with non-terminals.
If an internal node is labelled with a non-terminal A, and has n children with labels X1, Parse trees also have optional node numbers.
The above parse tree corresponds to a leftmost derivation. Traversing the tree can be done by three different forms of traversal.XML is a standardized markup language that defines a set of rules for encoding hierarchically structured documents in a human-readable text-based format.
XML is in widespread use, with documents ranging from very short and simple (such as SOAP queries) to multi-gigabyte documents (OpenStreetMap.
According to your grammar, this input would be completely static, but the parser would recurse four times into the section rule. An introduction to BNF and EBNF, two ways of formally defining languages.
Here is the generated image initiativeblog.com for the expression 14 + 2 * 3 - 6 / 2: Play with the utility a bit by passing it different arithmetic expressions and see what a parse tree looks like for a . In computer science, an LL parser is a top-down parser for a subset of context-free initiativeblog.com parses the input from Left to right, performing Leftmost derivation of the sentence..
An LL parser is called an LL(k) parser if it uses k tokens of lookahead when parsing a sentence.A grammar is called an LL(k) grammar if an LL(k) parser exists that can parse sentences belonging to the language. More often than not I write my parsers in PEG, which can later be translated into a hardcoded recursive descent directly.
There are few gotchas in converting a BNF to PEG, like, watching out for an order of parsing rules, but in general it is a mindless copy-pasting.