Practice example

Number recognition

C code generated by FLEX

Run your scanner to inspect the generated C code here.

KEEP THIS NEARBY

A little help with the patterns.

A scanner reads characters, matches patterns, and runs the action for each match.

Pattern	What it means
`[a-zA-Z]`	One letter
`[0-9]+`	One or more digits
`[a-zA-Z][a-zA-Z0-9]*`	Letter, then zero or more letters or digits
`* / +`	Zero or more / one or more
`a\|b`	Either a or b
`\.`	A literal dot
`.`	Any character except newline
`[ \t\n]+`	Whitespace

Shared lab mode

Write the PDF-style FLEX programs with <stdio.h>, named regex definitions, printf actions, and a simple main() calling yylex(). printf supports %s with yytext, %d with yyleng, and %% for a percent sign. Empty actions and ECHO are supported. General C declarations, additional functions, file access, networking, and system commands are unavailable. Each source/input is limited to 64 KB. Up to two scanners run at once, with 25 runs per minute per connection.

The three names to remember

yytext is the matched text. yyleng is its length. yylex() starts scanning.

How a rule wins

The longest match wins. If two matches have the same length, the rule written first wins. Put keywords before identifiers.

Three sections, two separators

Definitions
%%
Patterns and actions
%%
main() and helper functions

Examples follow your PDF. The email and comment hints are corrected; the medium example intentionally reports = as unknown. EOF is handled automatically. Accepted lab programs are rebuilt into a scanner with safe output actions before compilation.

01 / FEED YOUR SCANNER

Input

↳ Input is sent all at once. No terminal or EOF shortcut needed.

02 / SEE THE TOKENS

Output Ready

{ }

Your next “aha” starts here.

Run the example to see how your
scanner turns text into tokens.

Ctrl / ⌘ Enter to run

Expected sample output Compare your result

Build log

Your compilation steps will appear here.