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Regular Expression Flavor Comparison – Detailed comparison of the most popular regular expression flavors; Regexp Syntax Summary; Online Regular Expression Testing – with support for Java, JavaScript, .Net, PHP, Python and Ruby; Implementing Regular Expressions – series of articles by Russ Cox, author of RE2; Regular Expression Engines
Every regular expression can be written solely in terms of the Kleene star and set unions over finite words. This is a surprisingly difficult problem. As simple as the regular expressions are, there is no method to systematically rewrite them to some normal form. The lack of axiom in the past led to the star height problem.
However, parser generators for context-free grammars often support the ability for user-written code to introduce limited amounts of context-sensitivity. (For example, upon encountering a variable declaration, user-written code could save the name and type of the variable into an external data structure, so that these could be checked against ...
Another method [8] is to build the parse forest as you go, augmenting each Earley item with a pointer to a shared packed parse forest (SPPF) node labelled with a triple (s, i, j) where s is a symbol or an LR(0) item (production rule with dot), and i and j give the section of the input string derived by this node. A node's contents are either a ...
Shallow semantic parsing is concerned with identifying entities in an utterance and labelling them with the roles they play. Shallow semantic parsing is sometimes known as slot-filling or frame semantic parsing, since its theoretical basis comes from frame semantics, wherein a word evokes a frame of related concepts and roles.
The most established is lex, paired with the yacc parser generator, or rather some of their many reimplementations, like flex (often paired with GNU Bison). These generators are a form of domain-specific language, taking in a lexical specification – generally regular expressions with some markup – and emitting a lexer.
The parsing problem, checking whether a given word belongs to the language given by a context-free grammar, is decidable, using one of the general-purpose parsing algorithms: CYK algorithm (for grammars in Chomsky normal form) Earley parser; GLR parser; LL parser (only for the proper subclass of LL(k) grammars)
The first parser of this family to outperform a chart-based parser was the one by Muhua Zhu et al. in 2013, which took on the problem of length differences of different transition sequences due to unary constituency rules (a non-existent problem for dependency parsing) by adding a padding operation. [15]