Python regex dealing with "?"

Question

I know it's a pretty simple question. I happened to look at a regex example.

import re
pattern = r'^M?M?M?$'

s = "MDM"

re.search(pattern, s)

May I know why it doesn't match the string s? AFAIK, ? is to specify 0 or 1 occurence. It matches MMM though.

However the same string matches when the pattern is r'M?M?M?$' or r'^M?M?M?'. I am not getting what makes the difference here. Could someone please explain?

The `^` and `$` anchors together make the whole difference. The whole string must match. — Wiktor Stribiżew, Feb 23 '16 at 10:19
that pattern can only match first M, and regex evaluation hit $, so it does not match. — YOU, Feb 23 '16 at 10:24
`?` is a special character in python regex https://docs.python.org/2/howto/regex.html — gokul_uf, Feb 23 '16 at 10:25

score 2 · Accepted Answer · answered Feb 23 '16 at 10:27

2

r'^M?M?M?$' is the same as r'^M{0,3}$'. So, your pattern accepts '', 'M', 'MM', 'MMM' strings.

r'M?M?M?$' is the same as r'M{0,3}$ and, actually, accepts all strings, since there is always an empty part at the end of the string:

In [21]: pattern = r'M?M?M?$'

In [22]: re.search(pattern, 'A string without capital m at all')
Out[22]: <_sre.SRE_Match object; span=(33, 33), match=''>

answered Feb 23 '16 at 10:27

awesoon

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Nice. I didnt get this part though "actually, accepts all strings, since there is always an empty part at the end of the string:" – DineshKumar Feb 23 '16 at 13:11
Got it!!..Since we dont mention the beginning of a string, any string might end with 0 or 1 "M"s. Thats why it matches any string. Is that what u meant? – DineshKumar Feb 23 '16 at 13:25
@DineshKumar, exactly – awesoon Feb 23 '16 at 13:47

Maroun · Answer 2 · 2016-02-23T10:26:12.137

1

The regex

M?M?M?$

matches the last "M" in "MDM". But when you add ^ (beginning of string), it'll try to match from the beginning and it'll fail because M? matches 0 or 1 "M", but not a "D".

On the other regex:

^M?M?M?

The first "M" is matched.

edited Feb 23 '16 at 10:26

answered Feb 23 '16 at 10:19

Maroun

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Your explanation *`search` looks in the whole string, not from the beginning* does not sound right. It does search for a match from the beginning, but the match does not have to start at the beginning. – Wiktor Stribiżew Feb 23 '16 at 10:22
@WiktorStribiżew there's `match` and `search` in Python. [Here's](http://stackoverflow.com/questions/180986/what-is-the-difference-between-pythons-re-search-and-re-match) , that's what I tried to say, maybe I have to make it clearer a bit. – Maroun Feb 23 '16 at 10:24
I know the difference very well. It does not mean the `re.search` does not search for a match from the beginning of a string. Otherwise, `^M?` would not match the first `M`. – Wiktor Stribiżew Feb 23 '16 at 10:24
@WiktorStribiżew It *does* look, but as opposed to `match`, it doesn't attempt to match from the string beginning. – Maroun Feb 23 '16 at 10:25

score 1 · Answer 3 · answered Feb 23 '16 at 10:22

1

^ matches the start of a line. $ matches the end of the line.

so 'M?M?M?$' matches the last M in MDM and '^M?M?M?' matches the first M in MDM.

'^M?M?M?$' cannot match MDM because of the D in the middle that is not listed in your regex and the requirement to match the start of the line and the end of the line while there are 0, 1, 2 or 3 M in between.

answered Feb 23 '16 at 10:22

Simulant

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Thanks a lot. "the requirement to match the start of the line and the end of the line while there are 0, 1, 2 or 3 M in between." this statement nailed it. – DineshKumar Feb 23 '16 at 13:55

Python regex dealing with "?"

3 Answers3