7.10.4. PEG-04 — Email Validator

This tutorial builds an email address parser that extracts structured data into a daslang struct. You will learn:

not_set() for inverted character classes
Composing small pattern rules into larger grammars
Returning structs from parsers
Multiple character-set arguments to set()

7.10.4.1. Struct Returns

Parser rules can return any daslang type — including structs. Define the struct outside the grammar, then construct it in the rule body:

struct Email {
    user : string
    domain : string
}

7.10.4.2. The Grammar

A simplified email format: user@domain.tld.

def parse_email(input : string;
                blk : block<(val : Email; err : array<ParsingError>) : void>) {
    parse(input) {
        var email : Email
        rule("{+user_char}" as user, "@", "{+domain_char}" as domain, EOF) {
            return Email(user = user, domain = domain)
        }

        var user_char : void?
        rule(set('a'..'z', 'A'..'Z', '0'..'9', '.', '-', '_', '+')) {
            return null
        }

        var domain_char : void?
        rule(set('a'..'z', 'A'..'Z', '0'..'9', '-', '.')) {
            return null
        }
    }
}

The user_char and domain_char rules are void? pattern rules — they match characters but produce no value. Wrapping them in "{+...}" as name extracts the matched text.

7.10.4.3. Composing Pattern Rules

Small void? rules act as building blocks. Each matches a single character class; combining them with + (one-or-more) and text extraction ("{...}" as name) builds complex patterns from simple pieces.

This is the PEG equivalent of regex character classes like [a-zA-Z0-9._+-]+ — but type-checked at compile time.

7.10.4.4. not_set()

not_set() matches any character not in the given set. It is the complement of set():

not_set('\n', '\r')     // any character except newlines
not_set('"')            // any character except double-quote

7.10.4.5. Usage

parse_email("alice@example.com") $(val; err) {
    print("user={val.user}, domain={val.domain}\n")
    // user=alice, domain=example.com
}