@masala/parser/src/test/parsec/flow-regex-flags.spec.js

Masala Parser

import { describe, it, expect } from 'vitest'
import Streams from '../../lib/stream/index'
import { F } from '../../lib/parsec/index'

describe('F.regex – behaviour of individual RegExp flags', () => {
    /* ───────────────────────────── i : ignore-case ─────────────────────────── */
    it('i flag → matches despite case difference / no-i flag → fails', () => {
        // with i
        let stream = Streams.ofString('ABC')
        let parsing = F.regex(/abc/i).parse(stream)
        expect(parsing.isAccepted()).toBe(true)
        expect(parsing.value).toBe('ABC')
        expect(parsing.offset).toBe(3)

        // without i
        stream = Streams.ofString('ABC')
        parsing = F.regex(/abc/).parse(stream)
        expect(parsing.isAccepted()).toBe(false)
        expect(parsing.offset).toBe(0)
    })

    /* ───────────────────────────── s : dotAll ──────────────────────────────── */
    it('s flag → dot matches newline / no-s flag → fails', () => {
        // with s
        let stream = Streams.ofString('a\nb')
        let parsing = F.regex(/a.b/s).parse(stream)
        expect(parsing.isAccepted()).toBe(true)
        expect(parsing.value).toBe('a\nb')
        expect(parsing.offset).toBe(3)

        // without s
        stream = Streams.ofString('a\nb')
        parsing = F.regex(/a.b/).parse(stream)
        expect(parsing.isAccepted()).toBe(false)
    })

    /* ───────────────────────────── m : multiline anchors ───────────────────── */
    it('m flag → ^/$ see line breaks / no-m flag → fails mid-string', () => {
        const src = 'foo\nbar' // 'bar' starts at offset 4

        // with m
        let stream = Streams.ofString(src)
        let parsing = F.regex(/^bar$/m).parse(stream, 4)
        expect(parsing.isAccepted()).toBe(true)
        expect(parsing.value).toBe('bar')
        expect(parsing.offset).toBe(7) // 4 + 3

        // without m
        stream = Streams.ofString(src)
        parsing = F.regex(/^bar$/).parse(stream, 4)
        expect(parsing.isAccepted()).toBe(false)
    })

    /* ───────────────────────────── u : unicode escapes ─────────────────────── */
    it('u flag → \\u{…} escape recognised / no-u flag → fails', () => {
        const smile = '😀' // U+1F600 (surrogate pair)

        // with u
        let stream = Streams.ofString(smile)
        let parsing = F.regex(/\u{1F600}/u).parse(stream)
        expect(parsing.isAccepted()).toBe(true)
        expect(parsing.value).toBe(smile)
        expect(parsing.offset).toBe(smile.length) // 2 code units in UTF-16

        // without u
        stream = Streams.ofString(smile)
        parsing = F.regex(/\u{1F600}/).parse(stream)
        expect(parsing.isAccepted()).toBe(false)
    })

    it('matches only when the cursor is exactly on the pattern', () => {
        const stickyB = /b/y

        // cursor on the b  → accepted
        let stream = Streams.ofString('ab')
        let parsing = F.regex(stickyB).parse(stream, 1) // start at index 1
        expect(parsing.isAccepted()).toBe(true)
        expect(parsing.value).toBe('b')
        expect(parsing.offset).toBe(2) // consumed one char

        // cursor before the b → rejected (sticky prevents look-ahead)
        stream = Streams.ofString('ab')
        parsing = F.regex(stickyB).parse(stream, 0) // start at index 0
        expect(parsing.isAccepted()).toBe(false)
        expect(parsing.offset).toBe(0) // untouched on failure
    })
})