#Lex compiler online software#

#Lex compiler online software#

That is why several open source versions of both software were created. This means that their use was restricted, which left some users dissatisfied. The Open Source VersionsĪs mentioned, the initial versions were proprietary software. They were simply the best software available in their respective niches. Since Lex is used to generate lexers and Yacc to generate parsers, they were complementary and often used together. Yacc also became part of the POSIX standard. That is quite the level of dedication that you do not see in contemporary software. Johnson was a computer scientist that made sure that every trick to improve speed was theoretically sound and based in computer science. It was continually improved, and effectively also helped developing the theory of parsing. It started as a practical tool to help the development of the B Language. He wrote the latest version in C, although initially he wrote in B. Stephen Johnson developed Yacc during the early 1970s, writing (and rewriting) it many times between 19 as proprietary software. A parser deals with much more complicated problems than that of a lexer. For example, you can generate a parser that can take a IDENTIFIER (e.g., a variable), a EQUAL token, and a INT token and understand that together they form an assignment. Again, all you need to have is a grammar that describes the rules to follow.

Essentially a parser groups tokens (like the ones generated by Lex) into logical structures. Yacc is a parser generator, specifically a tool to generate LALR parsers. In fact it became part of the POSIX standard, essentially any respectable OS needed to have a tool like that. It was an important software, both for its quality and for the needs of development at that time. Mike Lesk and Eric Schmidt (who later became CEO of Google) developed it as proprietary software in 1975 in C. You can read more about the basics of parsing in our article: A Guide to Parsing: Algorithms and Terminology. It can be used for things like the recognition of elements of a programming language to perform syntax highlighting. A standalone lexer has few uses, because it basically just organizes set of characters into categories. So, for instance you can tell Lex that any grouping of digits (i.e., + characters) should be classified as an INT. All you need to have is a grammar that describes the rules to follow. A lexer accepts as input normal text and it organizes it in corresponding tokens. In case you do not know what a lexer is, these are the basics. Lex is a lexer generator, a tool to generate lexical analyzers. The Beginningsīoth software have an interesting history, although Yacc’s story looks like a soap opera. We briefly look at the history of these software. If you do not care about their history, you can go to the second part using the handy table of contents. The first part of this article explains the history of these two software, while the second one analyzes their flaws. It is a story both of weaknesses and of better alternatives.

The short version is that there are tools that are more flexible and productive, like ANTLR. With Bison our clients had trouble organizing large codebases and we found difficult to improve the efficiency of a parser without rewriting large part of the grammar.

So, why should you avoid them? Well, we found a few reasons based on our experience developing parsers for our clients.įor example, we had to work with existing lexers in flex and found difficult adding modern features, like Unicode support or making the lexer re-entrant (i.e., usable in many threads). For some people these are still the first software they think about when talking about parsing. Each of these software has more than 30 years of history, which is an achievement in itself. Lex and Yacc were the first popular and efficient lexers and parsers generators, flex and Bison were the first widespread open-source versions compatible with the original software.