Function parse_pattern 

pub fn parse_pattern(
    pattern: &str,
    custom_patterns: &HashMap<String, String>,
    allow_empty_delimited_default_string: bool,
    nesting_depth: usize,
) -> Result<ParsedPatternParts, FormatParseError>

Parse a format pattern string into regex parts, field specs, and names allow_empty_delimited_default_string: when false, default string fields always use .+? (used for the unanchored search regex so search/findall do not stop early).