在正则表达式Python中捕获$

Question

I'm trying to capture the dollar amount in a line:

example: blah blah blah (blah $23.32 blah) blah blac (blah) I want to capture "$23.32"

This is what I'm using: r'?([\\$][.*]+)'

I'm telling it to find one occurance of (...) with ? Then I tell it to find something which starts of with a "$" and any character which may come after (so I can get the decimal point also).

However, I get an error of error: nothing to repeat

Answer 1

The question mark at the start is the cause of the nothing to repeat error.

>>> import re
>>> re.compile(r'?')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mj/Development/venvs/stackoverflow-2.7/lib/python2.7/re.py", line 190, in compile
    return _compile(pattern, flags)
  File "/Users/mj/Development/venvs/stackoverflow-2.7/lib/python2.7/re.py", line 242, in _compile
    raise error, v # invalid expression
sre_constants.error: nothing to repeat

Match the dollar plus digits and dots:

r'\$[\d.]+'

Demo:

>>> re.search(r'\$[\d.]+', 'blah blah blah (blah $23.32 blah) blah blac (blah)').group()
'$23.32'

Answer 2

You should improve your basics about regular expressions. The error is due to the ? at the befinning. It's a quantifier and there is nothing before this quantifier. Your use of * and + makes also not much sense. Without knowing your exact requirements it's hard to propose a better solution, because there are too many problems with your regex.

Answer 3

Well, according to http://docs.python.org/2/library/re.html , [.*]+ would match .*..* , *....* , *.*.*. etc... As special characters lose their meanings in sets. Use [.\\d]+ or [.0-9]+ instead.

Answer 4

While suggestions for regexes are the way to go for more complicated patterns (and well worth the time to learn in general), there are other ways for simple cases. If I'm understanding the question, it seems that a little list comprehension, like:

x='blah blah blah (blah $23.32 blah) blah blac (blah)'
[i for i in x.split() if i.find('$') > -1]

would be a pretty concise way to go. It returns a list of strings.

['$23.32']

or, if there are no matches found,

[]