formalize a string with nested tags, which give each tag. '<1+<2+3>+4>+5' after formalized: 'LEVEL1<1+LEVEL2<2+3LEVEL1>+4LEVEL1>+5'

formalize

import re

text = '<1+<2+3>+4>+5'

def formalize(text, ltag='<', rtag='>', token='LEVEL'):
    pattern = r'({left})([^{left}{right}]*)|({right})([^{left}{right}]*)'.format(left=ltag, right=rtag)
    print pattern
    level = 0
    ret = []
    for match in re.finditer(pattern, text):
        g0, g1, g2, g3, g4 = match.group(0, 1, 2, 3, 4)
        if g1:
            level += 1
            ret.append('{token}{level}{tag}{text}'.format(token=token,level=level, tag=g1, text=g2 or ''))
        elif g3:
            level =+ 1
            ret.append('{token}{level}{tag}{text}'.format(token=token, level=level, tag=g3, text= g4 or ''))
    complete = True
    if level != 0:
        complete = False
    return ''.join(ret), complete

print formalize(text)