REXX- PARSE, DO, IF and SELECT

PARSE is the instruction that made REXX famous: one line can split a record by column positions or by delimiters. Combined with DO loops and IF/SELECT decisions, it handles most of the text-processing work you will ever do on the mainframe. Every example on this page is runnable TSO/E REXX.

How PARSE works

• PARSE takes a source string and a template describing how to split it into variables.
• The template has two kinds of patterns: positional (column numbers) and string patterns (literal delimiters).
• Parsing is always left to right. Each variable receives the text up to the next pattern.
• A period . in a template is a placeholder - it throws that piece away.

Positional templates

• A number in the template is an absolute column position, not a length. 6 name means "name starts at column 6".
• Each variable gets the text from its position up to the next position in the template.
• The last variable gets everything from its position to the end of the string.

Pattern (delimiter) templates

• A quoted string in the template is a literal pattern: parsing stops at that delimiter and the variable gets the text before it.
• This is the easiest way to split comma-separated or blank-separated data.
• Unquoted words in the template split on blanks, one word per variable.

PARSE variants: ARG, PULL, VAR, VALUE, SOURCE

• PARSE ARG - parse the parameters passed to the exec, preserving case (plain ARG uppercases).
• PARSE PULL - read a terminal/stack line without uppercasing it.
• PARSE VAR name - parse the value of a variable (shown above).
• PARSE VALUE expr WITH template - parse any expression, for example a function result.
• PARSE SOURCE - information about the environment; the first token is the language name.

DO loops

• DO n repeats n times. DO i = 1 TO 10 BY 2 is the classic counted loop.
• DO WHILE condition tests before each pass; DO UNTIL condition tests after, so it always runs at least once.
• DO FOREVER loops until you LEAVE it - always give it an exit path.
• LEAVE exits the innermost loop; ITERATE skips to the next pass.
• Every DO needs a matching END. You can name it: END i.

IF / THEN / ELSE

• IF condition THEN instruction - the workhorse decision. ELSE is optional.
• For several instructions, group them in a DO ... END block after THEN or ELSE.
• ELSE always pairs with the nearest IF, so use DO/END blocks when nesting.
• Comparisons: = \= > < >= <= (numeric when both sides are numbers), == \== for strict character comparison.

SELECT / WHEN / OTHERWISE

• SELECT tests a list of WHEN conditions from top to bottom; the first true one runs.
• OTHERWISE catches everything else - always code it so silent no-matches cannot hide bugs.
• SELECT ends with END. It replaces long IF/ELSE IF chains and is much easier to read.