Python Regex trying to find pattern that has two sets of optional characters

Question

I'm trying to use python regex on a pattern that has two sets of optional characters that may or may not be there. Below is what I'm trying to accomplish.

h becomes a when h is preceded by o but can also be preceded by a colon (:)
following the o and then maybe followed by f,y,r (f|y|r)

So this rule would be applied to the following patterns.

o:fh -> o:fa
ofh -> ofa
o:h -> o:a
oh -> oa

Below is what I'm trying.

re.sub(ur"o[(:|)][(f|y|r)]h", "o\1\2a", word);

I'm really struggling with the grouping and the two sets of optional characters : and (f|y|r) that may or may not be there. Any help is greatly appreciated. Thanks!

If h becomes a then why is h in the replacement?

hwnd
– hwnd

2014-08-09 20:35:13 +00:00
Commented Aug 9, 2014 at 20:35 — hwnd
– hwnd, Commented Aug 9, 2014 at 20:35

jwodder · Accepted Answer · 2014-08-09 20:43:59Z

2

Regex elements are made optional by following them with ?, not by enclosing them in brackets. The correct way (well, a correct way) to write your expression is:

re.sub(ur"o(:?[fyr]?)h", ur"o\1a", word)

Note that the replacement string has to be raw (r" ") so that the \1 won't be interpreted as character 0x01.

edited Aug 9, 2014 at 20:43

answered Aug 9, 2014 at 20:37

jwodder

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Comments

hwnd · Accepted Answer · 2014-08-09 22:06:44Z

1

Your syntax is incorrect, you are trying to use capturing groups inside of character classes. In simplest form, it lists the characters that may be matched inside square brackets ( matching any character from the list )

Regular expression visualization

You can simply use one group, following the characters you want to be optional with ?

>>> re.sub(ur'(o:?[yrf]?)h', ur'\1a', word)

Explanation:

(          # group and capture to \1:
  o        #   'o'
  :?       #   ':' (optional)
  [yrf]?   #   any character of: 'y', 'r', 'f' (optional)
)          # end of \1
h          # 'h'

You could use the regex module which supports variable-length lookbehind.

>>> import regex
>>> regex.sub(r'(?<=o:?[yrf]?)h', 'a', word)

edited Aug 9, 2014 at 22:06

answered Aug 9, 2014 at 21:03

hwnd

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