failed regex to match valid but non-lowercase name format

Error message

failed regex to match valid but non-lowercase name format

What it means

Same pattern as the lowercase check: the mixed-case regex ^[a-zA-Z][a-zA-Z0-9_]*$ in validate_name is compiled with .expect("failed regex to match valid but non-lowercase name format"). A panic here is an internal invariant violation, since the constant pattern is always valid. It exists to distinguish 'valid name but wrong case' from 'invalid format' when reporting InvalidProjectName.

Solutions

Rebuild or update gleam to the latest version
Inspect/update the regex dependency if a regression is suspected
File a Gleam issue with platform and build details if reproducible

Defensive patterns

Strategy: validation

Validate before calling

fn is_valid_but_not_lowercase(name: &str) -> bool {
    let c0 = name.chars().next();
    c0.map_or(false, |c| c.is_ascii_alphabetic())
        && name.chars().all(|c| c.is_ascii_alphanumeric() || c == '_')
        && name.chars().any(|c| c.is_ascii_uppercase())
}

Prevention

Prefer lowercase names to avoid the non-lowercase path entirely
Keep gleam builds reproducible and dependencies fresh
Report constant-regex panics upstream
Use the CLI suggestion when offered instead of crafting names

When it happens

Trigger: Called during validate_name (via suggest_valid_name or get_valid_project_name, e.g. `gleam new MyProject`) only if the regex engine fails to compile the fixed pattern — not caused by user input.

Common situations: Same as error 84: broken regex crate build, exotic target with insufficient stack, corrupted binary. Normally unreachable.

Related errors

AI-assisted analysis of gleam-lang/gleam@15b07c7830 (2026-09-14). Data as JSON: /api/errors/0b849f5b5227a66d. Report an issue: GitHub.

Appendix: source

Thrown at compiler-cli/src/new.rs:369

            reason: InvalidProjectNameReason::ErlangStandardLibraryModule,
        })
    } else if parse::lexer::string_to_keyword(name).is_some() {
        Err(Error::InvalidProjectName {
            name: name.to_string(),
            reason: InvalidProjectNameReason::GleamReservedWord,
        })
    } else if name == "gleam" {
        Err(Error::InvalidProjectName {
            name: name.to_string(),
            reason: InvalidProjectNameReason::GleamReservedModule,
        })
    } else if regex::Regex::new("^[a-z][a-z0-9_]*$")
        .expect("failed regex to match valid name format")
        .is_match(name)
    {
        Ok(())
    } else if regex::Regex::new("^[a-zA-Z][a-zA-Z0-9_]*$")
        .expect("failed regex to match valid but non-lowercase name format")
        .is_match(name)
    {
        Err(Error::InvalidProjectName {
            name: name.to_string(),
            reason: InvalidProjectNameReason::FormatNotLowercase,
        })
    } else {
        Err(Error::InvalidProjectName {
            name: name.to_string(),
            reason: InvalidProjectNameReason::Format,
        })
    }
}

fn suggest_valid_name(invalid_name: &str, reason: &InvalidProjectNameReason) -> Option<String> {
    match reason {
        InvalidProjectNameReason::GleamPrefix => match invalid_name.strip_prefix("gleam_") {
            Some(stripped) if invalid_name != "gleam_" => {

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