Ragel 6.10官方手册：状态机构建与详解

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Ragel 是一个强大的正则表达式到状态机编译器，专注于将复杂的文本模式转换为高效且易于维护的程序代码。这份官方使用手册，由 Adrian Thurston 编写，适用于 Ragel 版本 6.10，发布日期为 2017 年 3 月。它遵循 GNU General Public License（GPL）v2 或更高版本，确保了软件的开源性和自由传播。 1. **简介** - **抽象**：Ragel 提供了一种简洁的语法，允许程序员通过定义正则表达式规则来构建状态机，这种机器能够处理文本输入并根据匹配模式进行动作。这使得在 C/C++ 等语言中处理复杂的文本解析任务变得简单。 - **动机**：Ragel 的设计初衷是为了克服正则表达式的性能瓶颈，特别是当需要处理大量输入数据时，通过预编译状态机实现更高效的处理。 - **概述**：手册介绍了如何使用 Ragel 进行状态机的构造，包括块命名、机器定义、实例化以及引入外部 Ragel 代码的方式，同时还提及其他相关工作和开发进展。 2. **构建状态机** - **Ragel 状态机规格**：用户指南详细解释了如何书写 Ragel 语句，包括定义状态、转移条件、输入符号和输出动作。开发者需要明确地指定状态名，并按照特定的结构组织代码。 - **命名 Ragel 块**：为了提高代码的可读性和组织性，Ragel 鼓励用户使用有意义的名称来标识不同的状态块，如初始块、循环块或处理特定模式的块。 - **机器定义**：用户需要定义状态机的行为，包括输入字符与特定状态之间的映射，以及在状态转换时可能执行的操作。 - **机器实例化**：Ragel 产生的代码是高度可重用的，通过实例化可以创建多个处理不同输入的特定状态机实例。 - **引入 Ragel 代码**：手册说明了如何在已有项目中整合 Ragel 代码，通常通过编译后的 C/C++ 文件或头文件形式集成。 1.5 **相关工作和开发状态** - 该部分可能会讨论 Ragel 在同类工具中的优势和区别，比如与其他基于正则表达式的解析库相比，Ragel 在性能、灵活性和易用性方面的特点。同时，这部分会更新关于 RAGEL 的最新开发进度、改进和可能的未来方向。这份官方使用手册不仅提供了 Ragel 的核心功能介绍，还包含了实用的指南和最佳实践，对于希望利用 Ragel 来优化文本处理流程的开发者来说，是不可或缺的参考资料。阅读和理解这份文档，可以帮助用户充分利用 Ragel 的强大功能，编写出高效、清晰且易于维护的状态机代码。

Chapter 2

Constructing State Machines

2.1 Ragel State Machine Speciﬁcations

A Ragel input ﬁle consists of a program in the host language that contains embedded machine

speciﬁcations. Ragel normally passes input straight to output. When it sees a machine speciﬁcation

it stops to read the Ragel statements and possibly generate code in place of the speciﬁcation.

Afterwards it continues to pass input through. There can be any number of FSM speciﬁcations in

an input ﬁle. A multi-line FSM spec starts with %%{ and ends with }%%. A single-line FSM spec

starts with %% and ends at the ﬁrst newline.

While Ragel is looking for FSM speciﬁcations it does basic lexical analysis on the surrounding

input. It interprets literal strings and comments so a %% sequence in either of those will not trigger

the parsing of an FSM speciﬁcation. Ragel does not pass the input through any preprocessor nor

does it interpret preprocessor directives itself so includes, deﬁnes and ifdef logic cannot be used

to alter the parse of a Ragel input ﬁle. It is therefore not possible to use an #if 0 directive to

comment out a machine as is commonly done in C code. As an alternative, a machine can be

prevented from causing any generated output by commenting out write statements.

In Figure 2.1, a multi-line speciﬁcation is used to deﬁne the machine and single line speciﬁcations

are used to trigger the writing of the machine data and execution code.

#include <string.h>

#include <stdio.h>

%%{

machine foo;

main :=

( ’foo’ | ’bar’ )

0 @{ res = 1; };

}%%

%% write data;

int main( int argc, char **argv )

{

int cs, res = 0;

if ( argc > 1 ) {

char *p = argv[1];

char *pe = p + strlen(p) + 1;

%% write init;

%% write exec;

}

printf("result = %i\n", res );

return 0;

}

Figure 2.1: Parsing a command line argument.

CHAPTER 2. CONSTRUCTING STATE MACHINES 7

2.1.1 Naming Ragel Blocks

machine fsm_name;

The machine statement gives the name of the FSM. If present in a speciﬁcation, this statement

must appear ﬁrst. If a machine speciﬁcation does not have a name then Ragel uses the previous

speciﬁcation name. If no previous speciﬁcation name exists then this is an error. Because FSM

speciﬁcations persist in memory, a machine’s statements can be spread across multiple machine

speciﬁcations. This allows one to break up a machine across several ﬁles or draw in statements

that are common to multiple machines using the include statement.

2.1.2 Machine Deﬁnition

<name> = <expression>;

The machine deﬁnition statement associates an FSM expression with a name. Machine expres-

sions assigned to names can later be referenced in other expressions. A deﬁnition statement on its

own does not cause any states to be generated. It is simply a description of a machine to be used

later. States are generated only when a deﬁnition is instantiated, which happens when a deﬁnition

is referenced in an instantiated expression.

2.1.3 Machine Instantiation

<name> := <expression>;

The machine instantiation statement generates a set of states representing an expression. Each

instantiation generates a distinct set of states. The starting state of the instantiation is written

in the data section of the generated code using the instantiation name. If a machine named main

is instantiated, its start state is used as the speciﬁcation’s start state and is assigned to the cs

variable by the write init command. If no main machine is given, the start state of the last

machine instantiation to appear is used as the speciﬁcation’s start state.

From outside the execution loop, control may be passed to any machine by assigning the entry

point to the cs variable. From inside the execution loop, control may be passed to any machine

instantiation using fcall, fgoto or fnext statements.

2.1.4 Including Ragel Code

include FsmName "inputfile.rl";

The include statement can be used to draw in the statements of another FSM speciﬁcation.

Both the name and input ﬁle are optional, however at least one must be given. Without an FSM

name, the given input ﬁle is searched for an FSM of the same name as the current speciﬁcation.

Without an input ﬁle the current ﬁle is searched for a machine of the given name. If both are

present, the given input ﬁle is searched for a machine of the given name.

Ragel searches for included ﬁles from the location of the current ﬁle. Additional directories can

be added to the search path using the -I option.

2.1.5 Importing Deﬁnitions

import "inputfile.h";

CHAPTER 2. CONSTRUCTING STATE MACHINES 8

The import statement scrapes a ﬁle for sequences of tokens that match the following forms.

Ragel treats these forms as state machine deﬁnitions.

• name ’=’ number

• name ’=’ lit_string

• ’define’ name number

• ’define’ name lit_string

If the input ﬁle is a Ragel program then tokens inside any Ragel speciﬁcations are ignored. See

Section 5.8.5 for a description of exporting machine deﬁnitions.

Ragel searches for imported ﬁles from the location of the current ﬁle. Additional directories

can be added to the search path using the -I option.

2.2 Lexical Analysis of a Ragel Block

Within a machine speciﬁcation the following lexical rules apply to the input.

• The # symbol begins a comment that terminates at the next newline.

• The symbols "", ’’, //, [] behave as the delimiters of literal strings. Within them, the

following escape sequences are interpreted:

\0 \a \b \t \n \v \f \r

A backslash at the end of a line joins the following line onto the current. A backslash preceding

any other character removes special meaning. This applies to terminating characters and to

special characters in regular expression literals. As an exception, regular expression literals

do not support escape sequences as the operands of a range within a list. See the bullet on

regular expressions in Section 2.3.

• The symbols {} delimit a block of host language code that will be embedded into the machine

as an action. Within the block of host language code, basic lexical analysis of comments and

strings is done in order to correctly ﬁnd the closing brace of the block. With the exception

of FSM commands embedded in code blocks, the entire block is preserved as is for identical

reproduction in the output code.

• The pattern [+-]?[0-9]+ denotes an integer in decimal format. Integers used for specifying

machines may be negative only if the alphabet type is signed. Integers used for specifying

priorities may be positive or negative.

• The pattern 0x[0-9A-Fa-f]+ denotes an integer in hexadecimal format.

• The keywords are access, action, alphtype, getkey, write, machine and include.

• The pattern [a-zA-Z_][a-zA-Z_0-9]* denotes an identiﬁer.

• Any amount of whitespace may separate tokens.

2.3 Basic Machines

The basic machines are the base operands of regular language expressions. They are the smallest

unit to which machine construction and manipulation operators can be applied.

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