def extract_app_version(lines):
    """
    Extracts version string from array of lines (content of version.txt file)
    :param lines:
    :return: version string
    """
    for line in lines:
        parts = [elem.strip() for elem in line.split('=')]
        if len(parts) == 2 and parts[0].lower() == 'version_str':
            return parts[1].strip("'")
    return ''