cssx

cssx is a tiny, standalone Go library that parses extended CSS expressions and compiles them into a linear postfix pipeline IR.

It does not execute anything and does not depend on a browser, CDP, or DOM. It only provides:

• Parser: input string -> AST
• Builder: AST -> Pipeline IR

A separate package can then translate this IR into JS or any other runtime.

Installation

go get github.com/MontFerret/cssx

Quick Start

package main

import (
    "fmt"

    "github.com/MontFerret/cssx"
)

func main() {
    pipeline, err := cssx.Compile(":text(:first(h1))")
    if err != nil {
        panic(err)
    }

    for _, op := range pipeline.Ops {
        fmt.Printf("%s: %+v\n", op.Kind, op)
    }
}

Core Idea

Humans write nested pseudo calls like:

• :count(.product)
• :text(:first(h1))
• :attr("href", :first(a.cta))

cssx compiles them into a linear postfix pipeline:

• native CSS selector steps (Select)
• pseudo/function steps (Call)
• constants (Str, Num)

Example:

Input:

:text(:first(h1))

Pipeline:

[Select("h1"), Call("first",1), Call("text",1)]

Syntax

1) Plain CSS selector mode

If input does not start with :, it is treated as a plain selector.

.section .item

Compiles to:

[Select(".section .item")]

2) Expression mode (starts with :)

:text(:nth(2, .section .item))

Rules:

• Expr := SelectorExpr | CallExpr | StringLit | NumberLit
• CallExpr := ':' Ident '(' ArgList? ')'
• ArgList := Arg (',' Arg)*
• SelectorExpr is raw selector text until , or ) at the current nesting depth
• Strings support "..." and '...' with escapes
• Numbers are parsed with strconv.ParseFloat

3) Pipeline sugar (explicit delimiter)

.section .item >> :nth(2) >> :text()

Rules:

• >> splits pipeline only at top level (not inside quotes/brackets/parens)
• First segment: plain selector (must not start with :)
• Each following segment: must be a call :name(...)
• Zero-arg calls must still include parentheses: :text()

Compiles to:

[Select(".section .item"), Num(2), Call("nth",1), Call("text",0)]

Supported Examples

Plain selectors:

• .product -> [Select(".product")]
• .section .item -> [Select(".section .item")]

Single pseudo:

• :count(.product) -> [Select(".product"), Call("count",1)]
• :first(section) -> [Select("section"), Call("first",1)]

Nested pseudos:

• :text(:first(h1)) -> [Select("h1"), Call("first",1), Call("text",1)]
• :attr("href", :first(a.cta)) -> [Str("href"), Select("a.cta"), Call("first",1), Call("attr",2)]

Selectors with commas inside:

• :first(a[href*="x,y"]) -> [Select("a[href*="x,y"]"), Call("first",1)]

Mixed readability:

• :text(:nth(2, .section .item)) -> [Num(2), Select(".section .item"), Call("nth",2), Call("text",1)]

Pipeline sugar:

• .section .item >> :nth(2) >> :text() -> [Select(".section .item"), Num(2), Call("nth",1), Call("text",0)]

Public API

package cssx

// Parse parses input into an AST.
func Parse(input string) (AST, error)

// ParseToAST is an alias for Parse.
func ParseToAST(input string) (AST, error)

// BuildPipeline compiles an AST into a postfix pipeline IR.
func BuildPipeline(ast AST) (Pipeline, error)

// Compile parses input and builds a pipeline.
func Compile(input string) (Pipeline, error)

Key types:

type Pipeline struct {
    Ops []Op
}

type Op struct {
    Kind     OpKind
    Selector string
    Name     string
    Arity    int
    Str      string
    Num      float64
}

type ParseError struct {
    Message string
    Pos int // byte offset
}

Error Handling

All syntax errors return *ParseError with a byte offset position.

Common error cases:

• : (missing identifier)
• :text( (unterminated call)
• :text(:first(h1) (missing ))
• :attr("href" :first(a)) (missing comma)
• empty input (whitespace only)
• pipeline stage without :name(...)

Non-Goals

• No DOM, JS, or runtime execution
• No CSS selector validation
• No pseudo validation (names and arity are not checked)
• No external dependencies

License

See LICENSE.