Backslash in tag attribute value is escaped in Astro

[debiru]

Astro Info

Astro                    v5.7.11
Node                     v22.13.1
System                   Linux (x64)
Package Manager          pnpm
Output                   static
Adapter                  none
Integrations             myExt:integration

If this issue only occurs in one browser, which browser is a problem?

No response

Describe the Bug

If the following description is made in the astro file in src, the html file of the build result will look like the following.

.astro file in src directory

<input pattern="^example\.com$" />

.html file in dist directory

<input pattern="^example.com$" />

What's the expected result?

.html file in dist directory

<input pattern="^example\.com$" />

instead of

<input pattern="^example.com$" />

(The backslash is removed in the unexpected result)

Link to Minimal Reproducible Example

https://stackblitz.com/edit/withastro-astro-qxxakjka?file=src%2Fpages%2Findex.astro

Participation

• I am willing to submit a pull request for this issue.

[isVivek99]

i was trying to investigate this issue,
the componentFactory is returning the HTML where the regex is escaped and hence we are seeing this issue.
This happens in the complier itself, hence not sure if we can change something in the rendering logic,
also a workaround for @debiru would be to use this for now

<input pattern="^example\\.com$" />

since this is a valid escape sequence for javscript and would be outputted correctly.

[debiru]

@isVivek99 Thank you for your comment.

The existence of a workaround is not important; the critical issue is that the attribute value string as written in Astro is not output in HTML.

The current Astro behavior has destroyed the pattern attribute value.

This problem does not exist in React.

When writing in a plain HTML file, or when writing in a JSX file as React, backslash escaping does not need to be considered. However, Astro has to take it into account.

And the specification is probably unintentional. We believe that this behavior is a defect and should be corrected.

However, fixing this bug would be a destructive change for authors who write pattern attribute values based on this bug, and would generate incompatible HTML.

Perhaps a new flag needs to be established at https://astro.build/config that deals with whether or not the modified behavior should be enabled.

[MoustaphaDev]

This appears to be a bug on the js printer (and tsx printer). It doesn't occur on the AST printer.

If anyone wants to help out with this, I'd look around places we print quoted attributes and helper functions:

compiler/internal/printer/printer.go

Line 474 in 89c80fe

p.printTextWithSourcemap(encodeDoubleQuote(escapeInterpolation(escapeBackticks(attr.Val))), attr.ValLoc)

compiler/internal/printer/printer.go

Line 415 in 89c80fe

p.print(`"` + escapeDoubleQuote(a.Val) + `"`)

compiler/internal/printer/print-to-js.go

Line 461 in 89c80fe

p.print(`"` + escapeDoubleQuote(a.Val) + `"`)

I'd also look at the ast printer code to see why it works there

compiler/internal/printer/print-to-json.go

Line 233 in 89c80fe

if attr.Type == QuotedAttribute || attr.Type == TemplateLiteralAttribute {