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Parsing Techniques

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Parsing is a fundamental technique in programming that involves analyzing a string of symbols, either in natural language or computer languages, to understand its structure and meaning. In REXX, parsing is a powerful feature that allows you to manipulate and extract data from strings efficiently. This section will cover various parsing techniques in REXX, including basic parsing, template parsing, and advanced parsing methods.

  1. Basic Parsing

1.1 Parsing with the PARSE Instruction

The PARSE instruction in REXX is used to break down strings into smaller components. The basic syntax is:

PARSE [UPPER] [VAR] source [template]
  • UPPER: Converts the source string to uppercase before parsing.
  • VAR: Specifies that the source is a variable.
  • source: The string or variable to be parsed.
  • template: The pattern used to parse the source.

Example

/* Basic Parsing Example */
sourceString = "John Doe 1234"
PARSE VAR sourceString firstName lastName id

SAY "First Name:" firstName
SAY "Last Name:" lastName
SAY "ID:" id

Explanation:

  • sourceString contains the string to be parsed.
  • PARSE VAR sourceString firstName lastName id breaks the string into three parts based on spaces and assigns them to firstName, lastName, and id.

Output

First Name: John
Last Name: Doe
ID: 1234

  1. Template Parsing

Template parsing allows you to specify a more complex pattern for parsing strings. Templates can include literal strings, positional patterns, and variable patterns.

2.1 Positional Patterns

Positional patterns use placeholders to specify the position of data in the string.

Example

/* Positional Pattern Example */
sourceString = "John Doe 1234"
PARSE VAR sourceString firstName 1 lastName 6 id 10

SAY "First Name:" firstName
SAY "Last Name:" lastName
SAY "ID:" id

Explanation:

  • PARSE VAR sourceString firstName 1 lastName 6 id 10 specifies that firstName starts at position 1, lastName starts at position 6, and id starts at position 10.

Output

First Name: John
Last Name: Doe
ID: 1234

2.2 Literal Strings

Literal strings in templates allow you to match specific text within the source string.

Example

/* Literal String Example */
sourceString = "Name: John Doe, ID: 1234"
PARSE VAR sourceString "Name: " firstName " " lastName ", ID: " id

SAY "First Name:" firstName
SAY "Last Name:" lastName
SAY "ID:" id

Explanation:

  • The template "Name: " firstName " " lastName ", ID: " id matches the literal strings "Name: " and ", ID: " and extracts the values in between.

Output

First Name: John
Last Name: Doe
ID: 1234

  1. Advanced Parsing Techniques

3.1 Parsing with Delimiters

You can use delimiters to parse strings that contain specific characters separating the data.

Example

/* Parsing with Delimiters Example */
sourceString = "John,Doe,1234"
PARSE VAR sourceString firstName ',' lastName ',' id

SAY "First Name:" firstName
SAY "Last Name:" lastName
SAY "ID:" id

Explanation:

  • The delimiter ',' is used to separate the values in the string.

Output

First Name: John
Last Name: Doe
ID: 1234

3.2 Parsing with Multiple Delimiters

You can also parse strings with multiple delimiters by specifying them in the template.

Example

/* Parsing with Multiple Delimiters Example */
sourceString = "John;Doe|1234"
PARSE VAR sourceString firstName ';' lastName '|' id

SAY "First Name:" firstName
SAY "Last Name:" lastName
SAY "ID:" id

Explanation:

  • The delimiters ';' and '|' are used to separate the values in the string.

Output

First Name: John
Last Name: Doe
ID: 1234

  1. Practical Exercises

Exercise 1: Basic Parsing

Task: Parse the string "Alice Smith 5678" and extract the first name, last name, and ID.

Solution:

sourceString = "Alice Smith 5678"
PARSE VAR sourceString firstName lastName id

SAY "First Name:" firstName
SAY "Last Name:" lastName
SAY "ID:" id

Exercise 2: Template Parsing with Literal Strings

Task: Parse the string "User: Bob Brown, Code: 91011" and extract the first name, last name, and code.

Solution:

sourceString = "User: Bob Brown, Code: 91011"
PARSE VAR sourceString "User: " firstName " " lastName ", Code: " code

SAY "First Name:" firstName
SAY "Last Name:" lastName
SAY "Code:" code

Exercise 3: Parsing with Delimiters

Task: Parse the string "Charlie;Johnson|1213" and extract the first name, last name, and ID.

Solution:

sourceString = "Charlie;Johnson|1213"
PARSE VAR sourceString firstName ';' lastName '|' id

SAY "First Name:" firstName
SAY "Last Name:" lastName
SAY "ID:" id

Conclusion

In this section, we explored various parsing techniques in REXX, including basic parsing, template parsing, and advanced parsing with delimiters. Parsing is a powerful tool that allows you to manipulate and extract data from strings efficiently. By mastering these techniques, you can handle complex data structures and improve your REXX programming skills. In the next section, we will delve into interfacing with external programs, which will further enhance your ability to create robust and versatile REXX applications.

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