feat: add WebAssembly build for browser-based VCL tooling by acme · Pull Request #556 · ysugimoto/falco

acme · 2026-02-04T10:44:37Z

Compile Falco to WebAssembly, exposing a JavaScript API for VCL development in the browser.

New cmd/wasm/ package with entry point and API implementation
- FalcoVCL.parse(vcl) parses VCL to AST
- FalcoVCL.tokenize(vcl) tokenizes with semantic categories for syntax highlighting
- FalcoVCL.format(vcl, options) formats with configurable options
- FalcoVCL.lint(vcl, options) lints with scope-aware validation
Use https://github.com/dip-proto/go-pcre/tree/wasm for PCRE in both Go and Wasm
Makefile targets make wasm and make wasm_exec
Demo and tests
- wasm/index.html interactive demo page
- wasm/tests/ Vitest browser tests via Playwright

Compile Falco to WebAssembly, exposing a JavaScript API for VCL development in the browser. - New `cmd/wasm/` package with entry point and API implementation - `FalcoVCL.parse(vcl)` parses VCL to AST - `FalcoVCL.tokenize(vcl)` tokenizes with semantic categories for syntax highlighting - `FalcoVCL.format(vcl, options)` formats with configurable options - `FalcoVCL.lint(vcl, options)` lints with scope-aware validation - Build-tagged regex validation splits PCRE dependency for Wasm compatibility - `linter/regex.go` uses PCRE for native builds - `linter/regex_wasm.go` uses Go's `regexp` for Wasm (documented limitations) - Makefile targets `make wasm` and `make wasm_exec` - Demo and tests - `wasm/index.html` interactive demo page - `wasm/tests/` Vitest browser tests via Playwright

jedisct1 · 2026-02-04T11:00:24Z

Have you considered https://pkg.go.dev/go.arsenm.dev/pcre for PCRE ?

acme · 2026-02-04T11:19:58Z

Have you considered https://pkg.go.dev/go.arsenm.dev/pcre for PCRE ?

It looks like that depends on modernc.org/libc/, which won't work in Wasm?

jedisct1 · 2026-02-04T11:21:55Z

cmd/wasm/api.go

+		errors = append(errors, LintError{
+			Severity: "error",
+			Message:  l.FatalError.Error.Error(),
+			Line:     1,


Can't Fatal errors get a line/position?

So they can! 4648db0 implements that.

jedisct1 · 2026-02-04T11:42:00Z

Have you considered https://pkg.go.dev/go.arsenm.dev/pcre for PCRE ?

It looks like that depends on modernc.org/libc/, which won't work in Wasm?

Indeed.

Maybe that package could work on Wasm with a minimal libc shim?

PCRE is a common requirement, so this would be very useful for Go and WebAssembly users in general, and it would also avoid an inconsistency between Falco running on Wasm and non-Wasm environments.

jedisct1 · 2026-02-04T11:52:17Z

The CGO-free PCRE can work on WebAssembly: https://github.com/dip-proto/go-pcre/tree/wasm

- Replace `go.elara.ws/pcre` with `dip-proto/go-pcre` fork that supports js/wasm target - Remove `regex_wasm.go` fallback that used Go's limited `regexp` package - Consolidate into single `regex.go` without build tags - Enables lookahead/lookbehind regex validation in browser environment

acme · 2026-02-04T13:33:23Z

The CGO-free PCRE can work on WebAssembly: https://github.com/dip-proto/go-pcre/tree/wasm

Nice! 0d5280b uses that instead.

- Parse `FatalError.Error` as `ParseError` to get actual token location

- Add explicit yaml.v3 dependency to go.mod

jedisct1 · 2026-02-06T16:05:04Z

wasm/README.md

+## Limitations
+
+**Regex validation**: The native Falco CLI uses PCRE for regex validation, but PCRE's native code cannot run in WebAssembly. The Wasm build uses Go's standard `regexp` package instead, which lacks support for some PCRE features:
+
+- Lookahead (`(?=...)`, `(?!...)`)
+- Lookbehind (`(?<=...)`, `(?<!...)`)
+- Atomic groups (`(?>...)`)
+- Possessive quantifiers (`*+`, `++`, `?+`)
+
+VCL patterns using these features will pass validation in the WASM build but may fail in production or when using the native CLI.


That section can be removed.

Thanks, removed as part of fe56ea0

ysugimoto

I was thinking how we provide as wasm... and then this PR is great solution, thanks.

Probably we should release wasm binary at the same time with common binaries (should be done on another PR)

ysugimoto · 2026-02-10T11:26:37Z

linter/regex.go

+package linter
+
+import (
+	regexp "go.elara.ws/pcre"


From discussion of #556 (comment), can we replace to github.com/dip-proto/go-pcre ?

ysugimoto · 2026-02-10T11:27:25Z

go.mod

 	modernc.org/memory v1.2.0 // indirect
 )
+
+replace go.elara.ws/pcre => github.com/dip-proto/go-pcre v0.0.0-20260204122309-dcbff9cb6240


No longer we need replace section due to you have removed this package?

Hmm, I'm not sure. I think we still need it as @jedisct1's fork identifies as go.elara.ws/pcre https://github.com/dip-proto/go-pcre/blob/wasm/go.mod#L1C8-L2C1 ?

ysugimoto · 2026-02-10T11:31:32Z

wasm/index.html

+    }
+
+    function escapeHtml(str) {
+      return str.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');


To escape completely, I think we need to escape ' and " for HTML attribute also.

79fa83c renames the function and adds a comment to make it clearer what it is escaping

- Clarify that the function only escapes characters needed for safe text node insertion, not attribute values

jedisct1 reviewed Feb 4, 2026

View reviewed changes

acme added 2 commits February 4, 2026 13:32

fix(linter): correct scope iteration boundary in ScopesString

2cfe23b

acme and others added 5 commits February 4, 2026 13:39

fix(wasm): extract line/position from fatal lint errors

4648db0

- Parse `FatalError.Error` as `ParseError` to get actual token location

chore(deps): add gopkg.in/yaml.v3 as direct dependency

87e6deb

- Add explicit yaml.v3 dependency to go.mod

Merge branch 'main' into acme/add-wasm

25b54b6

Merge branch 'main' into acme/add-wasm

dcb37ac

Merge branch 'main' into acme/add-wasm

b9d7b87

jedisct1 reviewed Feb 6, 2026

View reviewed changes

acme added 2 commits February 6, 2026 16:30

Remove old limitations

fe56ea0

Merge branch 'acme/add-wasm' of github.com:acme/falco into acme/add-wasm

02c749c

ysugimoto reviewed Feb 10, 2026

View reviewed changes

acme added 3 commits February 11, 2026 14:06

refactor: rename escapeHtml to escapeHtmlText

79fa83c

- Clarify that the function only escapes characters needed for safe text node insertion, not attribute values

Merge branch 'main' into acme/add-wasm

7abb2fb

Merge branch 'main' into acme/add-wasm

6452a5e

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