[swift5] fix: keep pattern regexes valid Swift string literals

[wiebren]

Any schema pattern that contains a / currently generates swift5 code that does not
compile. Fixes #15604 (open since 6.3.0, still present on master).

Given:

url:
  type: string
  pattern: 'http(s)?:\/\/x'
plainSlash:
  type: string
  pattern: '[a-z/]+'

master emits:

public static let urlRule = StringRule(minLength: nil, maxLength: nil, pattern: "/http(s)?:\/\/x/")
public static let plainSlashRule = StringRule(minLength: nil, maxLength: nil, pattern: "/[a-z\/]+/")

and swiftc stops with error: invalid escape sequence in literal on every \/ — \/ is
not a valid escape in a Swift string literal. The plainSlash case shows the backslash is
not coming from the spec (its pattern contains no backslash at all): it is introduced by the
generator.

The cause

DefaultCodegen.addRegularExpressionDelimiter wraps an undelimited pattern in /.../ and
escapes every inner slash while doing so:

return "/" + pattern.replaceAll("/", "\\\\/") + "/";

That Perl-style delimiter escaping is fine for languages that embed the pattern in a /.../
regex literal, but swift5's modelObject.mustache embeds it in a double-quoted string
literal, where \/ is a compile error.

The delimiters themselves are also unwanted for this generator: the generated
Validation.swift hands rule.pattern straight to NSRegularExpression(pattern:), which
has no delimiter syntax — ICU treats the wrapping / as literal characters to match.

The fix

Swift5ClientCodegen overrides toRegularExpression to escape the pattern for the string
literal without adding delimiters — the same override AbstractKotlinCodegen already uses
for the same reason (its patterns also go into a plain string literal):

@Override
public String toRegularExpression(String pattern) {
    return escapeText(pattern);
}

After the fix the repro above generates

public static let urlRule = StringRule(minLength: nil, maxLength: nil, pattern: "http(s)?://x")
public static let plainSlashRule = StringRule(minLength: nil, maxLength: nil, pattern: "[a-z/]+")

which swiftc -parse accepts, and NSRegularExpression("http(s)?://x") now actually
matches https://x (with the delimiters it could not, because of the leading literal /).

A pattern that already carries delimiters in the spec — like the petstore's /[a-z]/i — is
left exactly as it was, so no sample output changes: ./bin/generate-samples.sh ./bin/configs/swift5-*.yaml and ./bin/utils/export_docs_generators.sh both produce an
empty diff. The only pattern in the swift5 samples is the already-delimited /[a-z]/i.

Tests

Swift5ClientCodegenTest#testToRegularExpressionRemainsValidInSwiftStringLiteral covers a
pattern with bare slashes, one with JSON-style \/ escapes, one with regex backslash
escapes, and one that already carries delimiters. It fails on master (master returns
"/http(s)?:\/\/x/" for the first case) and passes with the fix; the full swift5 test
classes (40 tests) pass.

The swift6 generator inherits the same defect from the same DefaultCodegen default and the
identical override would fix it there too — happy to add it here or as a follow-up PR,
whichever you prefer.

PR checklist

• Read the contribution guidelines.
• Built the project and updated samples (./bin/generate-samples.sh ./bin/configs/swift5-*.yaml and ./bin/utils/export_docs_generators.sh — both
  produced no diff, see above).
• Technical committee: @4brunu

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Generated with Claude Code

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Summary by cubic

Fixes swift5 codegen so schema patterns containing / generate Swift that compiles (issue #15604) and reach NSRegularExpression without unwanted delimiters.

Bug Fixes

• toRegularExpression now escapes the pattern for the Swift string literal without adding /.../ delimiters, matching AbstractKotlinCodegen.
• Without the fix, swiftc rejected the \/ escapes and NSRegularExpression matched the literal delimiters, so http(s)?://x never matched https://x.
• No sample output changes (the only swift5 sample pattern already carries delimiters), and a new test covers slashes, JSON-style escapes, regex backslashes, and pre-delimited patterns.
• swift6 inherits the same defect; the same override applies there as a follow-up.

^{Written for commit 4a44ee0. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}