Function validate_substitution_modifiers 

pub fn validate_substitution_modifiers(
    modifiers_str: &str,
) -> Result<String, char>

Validate substitution modifiers and return an error if any are invalid

Valid Perl substitution modifiers include:

• Core modifiers: g, i, m, s, x, o, e, r
• Charset modifiers (Perl 5.14+): a, d, l, u
• Additional modifiers: n (5.22+), p, c

Arguments

• modifiers_str - The raw modifier string following the substitution operator

Returns

• Ok(String) - The validated modifiers if all are valid
• Err(char) - The first invalid modifier character encountered

Examples

assert!(validate_substitution_modifiers("gi").is_ok());
assert!(validate_substitution_modifiers("gia").is_ok());  // 'a' for ASCII mode
assert!(validate_substitution_modifiers("giz").is_err()); // 'z' is invalid