<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def regex_replace(txt, rgx, val, ignorecase=False, multiline=False):
    r'''
    Searches for a pattern and replaces with a sequence of characters.

    .. code-block:: jinja

        {% set my_text = 'lets replace spaces' %}
        {{ my_text | regex_replace('\s+', '__') }}

    will be rendered as:

    .. code-block:: text

        lets__replace__spaces
    '''
    flag = 0
    if ignorecase:
        flag |= re.I
    if multiline:
        flag |= re.M
    compiled_rgx = re.compile(rgx, flag)
    return compiled_rgx.sub(val, txt)