Reputation: 83
I am looking for a Python regex for a variable phrase with the following properties:
(For the sake of example, let's assume the variable phrase here is taking the value and
. But note that I need to do this in a way that the thing playing the role of and
can be passed in as a variable which I'll call phrase
.)
Should match: this_and
, this.and
, (and)
, [and]
, and^
, ;And
, etc.
Should not match: land
, andy
This is what I tried so far (where phrase
is playing the role of and
):
pattern = r"\b " + re.escape(phrase.lower()) + r"\b"
This seems to work for all my requirements except that it does not match words with underscores e.g. \_hello
, hello\_
, hello_world
.
Edit: Ideally I would like to use the standard library re module rather than any external packages.
Upvotes: 5
Views: 2713
Reputation: 627607
You may use
r'(?<![^\W_])and(?![^\W_])'
See the regex demo. Compile with the re.I
flag to enable case insensitive matching.
Details
(?<![^\W_])
- the preceding char should not be a letter or digit charand
- some keyword(?![^\W_])
- the next char cannot be a letter or digitimport re
strs = ['this_and', 'this.and', '(and)', '[and]', 'and^', ';And', 'land', 'andy']
phrase = "and"
rx = re.compile(r'(?<![^\W_]){}(?![^\W_])'.format(re.escape(phrase)), re.I)
for s in strs:
print("{}: {}".format(s, bool(rx.search(s))))
Output:
this_and: True
this.and: True
(and): True
[and]: True
and^: True
;And: True
land: False
andy: False
Upvotes: 6
Reputation: 506
Here is a regex that might solve it:
Regex
(?<=[\W_]+|^)and(?=[\W_]+|$)
Example
# import regex
string = 'this_And'
test = regex.search(r'(?<=[\W_]+|^)and(?=[\W_]+|$)', string.lower())
print(test.group(0))
# prints 'and'
# No match
string = 'Andy'
test = regex.search(r'(?<=[\W_]+|^)and(?=[\W_]+|$)', string.lower())
print(test)
# prints None
strings = [ "this_and", "this.and", "(and)", "[and]", "and^", ";And"]
[regex.search(r'(?<=[\W_]+|^)and(?=[\W_]+|$)', s.lower()).group(0) for s in strings if regex.search(r'(?<=[\W_]+|^)and(?=[\W_]+|$)', s.lower())]
# prints ['and', 'and', 'and', 'and', 'and', 'and']
Explanation
[\W_]+
means we accept before (?<=
) or after (?=
) and
only non-word symbols except the underscore _
(a word symbol that) is accepted. |^
and |$
allow matches to lie at the edge of the string.
Edit
As mentioned in my comment, the module regex
does not yield errors with variable lookbehind lengths (as opposed to re
).
# This works fine
# import regex
word = 'and'
pattern = r'(?<=[\W_]+|^){}(?=[\W_]+|$)'.format(word.lower())
string = 'this_And'
regex.search(pattern, string.lower())
However, if you insist on using re
, then of the top of my head I'd suggest splitting the lookbehind in two (?<=[\W_])and(?=[\W_]+|$)|^and(?=[\W_]+|$)
that way cases where the string starts with and
are captured as well.
# This also works fine
# import re
word = 'and'
pattern = r'(?<=[\W_]){}(?=[\W_]+|$)|^{}(?=[\W_]+|$)'.format(word.lower(), word.lower())
string = 'this_And'
re.search(pattern, string.lower())
Upvotes: 2