I'm learning about regular expression. I don't know how to combine different regular expression to make a single generic regular expression.
I want to write a single regular expression which works for multiple cases. I know this is can be done with naive approach by using or " | " operator.
I don't like this approach. Can anybody tell me better approach?
You need to compile all your regex functions. Check this example:
import re
re1 = r'\d+\.\d*[L][-]\d*\s[A-Z]*[/]\d*'
re2 = '\d*[/]\d*[A-Z]*\d*\s[A-Z]*\d*[A-Z]*'
re3 = '[A-Z]*\d+[/]\d+[A-Z]\d+'
re4 = '\d+[/]\d+[A-Z]*\d+\s\d+[A-Z]\s[A-Z]*'
sentences = [string1, string2, string3, string4]
for sentence in sentences:
generic_re = re.compile("(%s|%s|%s|%s)" % (re1, re2, re3, re4)).findall(sentence)
To findall with an arbitrary series of REs all you have to do is concatenate the list of matches which each returns:
re_list = [
'\d+\.\d*[L][-]\d*\s[A-Z]*[/]\d*', # re1 in question,
...
'\d+[/]\d+[A-Z]*\d+\s\d+[A-z]\s[A-Z]*', # re4 in question
]
matches = []
for r in re_list:
matches += re.findall( r, string)
For efficiency it would be better to use a list of compiled REs.
Alternatively you could join the element RE strings using
generic_re = re.compile( '|'.join( re_list) )
I see lots of people are using pipes, but that seems to only match the first instance. If you want to match all, then try using lookaheads.
Example:
>>> fruit_string = "10a11p"
>>> fruit_regex = r'(?=.*?(?P<pears>\d+)p)(?=.*?(?P<apples>\d+)a)'
>>> re.match(fruit_regex, fruit_string).groupdict()
{'apples': '10', 'pears': '11'}
>>> re.match(fruit_regex, fruit_string).group(0)
'10a,11p'
>>> re.match(fruit_regex, fruit_string).group(1)
'11'
(?= ...) is a look ahead:
Matches if ... matches next, but doesn’t consume any of the string. This is called a lookahead assertion. For example, Isaac (?=Asimov) will match 'Isaac ' only if it’s followed by 'Asimov'.
.*?(?P<pears>\d+)p
find a number followed a p anywhere in the string and name the number "pears"
You might not need to compile both regex patterns. Here is a way, let's see if it works for you.
>>> import re
>>> text = 'aaabaaaabbb'
>>> A = 'aaa'
>>> B = 'bbb'
>>> re.findall(A+B, text)
['aaabbb']
>>>
further read read_doc
If you need to squash multiple regex patterns together the result can be annoying to parse--unless you use P<?> and .groupdict() but doing that can be pretty verbose and hacky. If you only need a couple matches then doing something like the following could be mostly safe:
bucket_name, blob_path = tuple(item for item in matches.groups() if item is not None)
Related
I do not consider myself a newbie in regex, but I seem to have found a problem that stumped me (it's also Friday evening, so brain not at peak performance).
I am trying to substitute a place-holder inside a string with some other value. I am having great difficulty getting a syntax that behaves the way I want.
My place-holder has this format: {swap}
I want it to capture and replace these:
{swap} # NewValue
x{swap}x # xNewValuex
{swap}x # NewValuex
x{swap} # xNewValue
But I want it to NOT match these:
{{swap}} # NOT {NewValue}
x{{swap}}x # NOT x{NewValue}x
{{swap}}x # NOT {NewValue}x
x{{swap}} # NOT x{NewValue}
In all of the above, x can be any string, of any length, be it "word" or not.
I'm trying to do this using python3's re.sub() but anytime I satisfy one subset of criteria I lose another in the process. I'm starting to think it might not be possible to do in a single command.
Cheers!
If you're able to use the newer regex module, you can use (*SKIP)(*FAIL):
{{.*?}}(*SKIP)(*FAIL)|{.*?}
See a demo on regex101.com.
Broken down, this says:
{{.*?}}(*SKIP)(*FAIL) # match any {{...}} and "throw them away"
| # or ...
{.*?} # match your desired pattern
In Python this would be:
import regex as re
rx = re.compile(r'{{.*?}}(*SKIP)(*FAIL)|{.*?}')
string = """
{swap}
x{swap}x
{swap}x
x{swap}
{{swap}}
x{{swap}}x
{{swap}}x
x{{swap}}"""
string = rx.sub('NewValue', string)
print(string)
This yields:
NewValue
xNewValuex
NewValuex
xNewValue
{{swap}}
x{{swap}}x
{{swap}}x
x{{swap}}
For the sake of completeness, you can also achieve this with Python's own re module but here, you'll need a slightly adjusted pattern as well as a replacement function:
import re
rx = re.compile(r'{{.*?}}|({.*?})')
string = """
{swap}
x{swap}x
{swap}x
x{swap}
{{swap}}
x{{swap}}x
{{swap}}x
x{{swap}}"""
def repl(match):
if match.group(1) is not None:
return "NewValue"
else:
return match.group(0)
string = rx.sub(repl, string)
print(string)
Use negative lookahead and lookbehind:
s1 = "x{swap}x"
s2 = "x{{swap}}x"
pattern = r"(?<!\{)\{[^}]+\}(?!})"
re.sub(pattern, "foo", s1)
#'xfoox'
re.sub(pattern, "foo", s2)
#'x{{swap}}x'
I want to use regex to search in a file for this expression:
time:<float> s
I only want to get the float number.
I'm learning about regex, and this is what I did:
astr = 'lalala time:1.5 s\n'
p = re.compile(r'time:(\d+).*(\d+)')
m = p.search(astr)
Well, I get time:1.5 from m.group(0)
How can I directly just get 1.5 ?
I'm including some extra python-specific materiel since you said you're learning regex. As already mentioned the simplest regex for this would certainly be \d+\.\d+ in various commands as described below.
Something that threw me off with python initially was getting my head around the return types of various re methods and when to use group() vs. groups().
There are several methods you might use:
re.match()
re.search()
re.findall()
match() will only return an object if the pattern is found at the beginning of the string.
search() will find the first pattern and top.
findall() will find everything in the string.
The return type for match() and search() is a match object, __Match[T], or None, if a match isn't found. However the return type for findall() is a list[T]. These different return types obviously have ramifications for how you get the values out of your match.
Both match and search expose the group() and groups() methods for retrieving your matches. But when using findall you'll want to iterate through your list or pull a value with an enumerator. So using findall:
>>>import re
>>>easy = re.compile(r'123')
>>>matches = easy.findall(search_me)
>>>for match in matches: print match
123
If you're using search() or match(), you'll want to use .group() or groups() to retrieve your match depending on how you've set up your regular expression.
From the documentation, "The groups() method returns a tuple containing the strings for all the subgroups, from 1 up to however many there are."
Therefore if you have no groups in your regex, as shown in the following example, you wont get anything back:
>>>import re
>>>search_me = '123abc'
>>>easy = re.compile(r'123')
>>>matches = easy.search(search_me)
>>>print matches.groups()
()
Adding a "group" to your regular expression enables you to use this:
>>>import re
>>>search_me = '123abc'
>>>easy = re.compile(r'(123)')
>>>matches = easy.search(search_me)
>>>print matches.groups()
('123',)
You don't have to specify groups in your regex. group(0) or group() will return the entire match even if you don't have anything in parenthesis in your expression. --group() defaults to group(0).
>>>import re
>>>search_me = '123abc'
>>>easy = re.compile(r'123')
>>>matches = easy.search(search_me)
>>>print matches.group(0)
123
If you are using parenthesis you can use group to match specific groups and subgroups.
>>>import re
>>>search_me = '123abc'
>>>easy = re.compile(r'((1)(2)(3))')
>>>matches = easy.search(search_me)
>>>print matches.group(1)
>>>print matches.group(2)
>>>print matches.group(3)
>>>print matches.group(4)
123
1
2
3
I'd like to point as well that you don't have to compile your regex unless you care to for reasons of usability and/or readability. It won't improve your performance.
>>>import re
>>>search_me = '123abc'
>>>#easy = re.compile(r'123')
>>>#matches = easy.search(search_me)
>>>matches = re.search(r'123', search_me)
>>>print matches.group()
Hope this helps! I found sites like debuggex helpful while learning regex. (Although sometimes you have to refresh those pages; I was banging my head for a couple hours one night before I realized that after reloading the page my regex worked just fine.) Lately I think you're served just as well by throwing sandbox code into something like wakari.io, or an IDE like PyCharm, etc., and observing the output. http://www.rexegg.com/ is also a good site for general regex knowledge.
You could do create another group for that. And I would also change the regex slightly to allow for numbers that don't have a decimal separator.
re.compile(r'time:((\d+)(\.?(\d+))?')
Now you can use group(1) to capture the match of the floating point number.
I think the regex you actually want is something more like:
re.compile(r'time:(\d+\.\d+)')
or even:
re.compile(r'time:(\d+(?:\.\d+)?)') # This one will capture integers too.
Note that I've put the entire time into 1 grouping. I've also escaped the . which means any character in regex.
Then, you'd get 1.5 from m.group(1) -- m.group(0) is the entire match. m.group(1) is the first submatch (parenthesized grouping), m.group(2) is the second grouping, etc.
example:
>>> import re
>>> p = re.compile(r'time:(\d+(?:\.\d+)?)')
>>> p.search('time:34')
<_sre.SRE_Match object at 0x10fa77d50>
>>> p.search('time:34').group(1)
'34'
>>> p.search('time:34.55').group(1)
'34.55'
I need to find the value of "taxid" in a large number of strings similar to one given below. For this particular string, the 'taxid' value is '9606'. I need to discard everything else. The "taxid" may appear anywhere in the text, but will always be followed by a ":" and then number.
score:0.86|taxid:9606(Human)|intact:EBI-999900
How to write regular expression for this in python.
>>> import re
>>> s = 'score:0.86|taxid:9606(Human)|intact:EBI-999900'
>>> re.search(r'taxid:(\d+)', s).group(1)
'9606'
If there are multiple taxids, use re.findall, which returns a list of all matches:
>>> re.findall(r'taxid:(\d+)', s)
['9606']
for line in lines:
match = re.match(".*\|taxid:([^|]+)\|.*",line)
print match.groups()
If I have a string like this:
*select 65* description
How could I extract the bit after the asterisk and the number using regex in Python? I need something that would yield select and 65 from the above string.
All of them follow this convention:
*[lowercase specifier] [integer]* description
You could use this regular expression:
^\*([a-z]+)\s+([0-9]+)\*
In Python, you can match regular expressions with the re module. Thus:
import re
my_string = """*select 65* description"""
match = re.match(r"^\*([a-z]+)\s+([0-9]+)\*", my_string)
specifier = match.group(1)
integer = int(match.group(2))
import re
and then either
m = re.match(r"^\*([a-z]+)\s+([0-9]+)\*\s+(.*)", "*select 65* description")
print m.groups()
or
r = re.compile(r"^\*([a-z]+)\s+([0-9]+)\*\s+(.*)")
m = r.match("*select 65* description")
print m.groups()
depending on the number of matches you want to make. The former is better suited for one or few matches, the latter better for many, because the regex is compiled in a form which is better suited for multiple executions.
Python's regex library is powerful, but I'm personally fond of using split() for lighter-weight problems:
>>> s = "*select 65* description"
>>> s.split('*')
['', 'select 65', ' description']
>>> s.split('*')[1].split()
['select', '65']
In Perl it is possible to do something like this (I hope the syntax is right...):
$string =~ m/lalala(I want this part)lalala/;
$whatIWant = $1;
I want to do the same in Python and get the text inside the parenthesis in a string like $1.
If you want to get parts by name you can also do this:
>>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Malcom Reynolds")
>>> m.groupdict()
{'first_name': 'Malcom', 'last_name': 'Reynolds'}
The example was taken from the re docs
See: Python regex match objects
>>> import re
>>> p = re.compile("lalala(I want this part)lalala")
>>> p.match("lalalaI want this partlalala").group(1)
'I want this part'
import re
astr = 'lalalabeeplalala'
match = re.search('lalala(.*)lalala', astr)
whatIWant = match.group(1) if match else None
print(whatIWant)
A small note: in Perl, when you write
$string =~ m/lalala(.*)lalala/;
the regexp can match anywhere in the string. The equivalent is accomplished with the re.search() function, not the re.match() function, which requires that the pattern match starting at the beginning of the string.
import re
data = "some input data"
m = re.search("some (input) data", data)
if m: # "if match was successful" / "if matched"
print m.group(1)
Check the docs for more.
there's no need for regex. think simple.
>>> "lalala(I want this part)lalala".split("lalala")
['', '(I want this part)', '']
>>> "lalala(I want this part)lalala".split("lalala")[1]
'(I want this part)'
>>>
import re
match = re.match('lalala(I want this part)lalala', 'lalalaI want this partlalala')
print match.group(1)
import re
string_to_check = "other_text...lalalaI want this partlalala...other_text"
p = re.compile("lalala(I want this part)lalala") # regex pattern
m = p.search(string_to_check) # use p.match if what you want is always at beginning of string
if m:
print m.group(1)
In trying to convert a Perl program to Python that parses function names out of modules, I ran into this problem, I received an error saying "group" was undefined. I soon realized that the exception was being thrown because p.match / p.search returns 0 if there is not a matching string.
Thus, the group operator cannot function on it. So, to avoid an exception, check if a match has been stored and then apply the group operator.
import re
filename = './file_to_parse.py'
p = re.compile('def (\w*)') # \w* greedily matches [a-zA-Z0-9_] character set
for each_line in open(filename,'r'):
m = p.match(each_line) # tries to match regex rule in p
if m:
m = m.group(1)
print m