If any strings in a list match regex

Question

I need to check if any of the strings in a list match a regex. If any do, I want to continue. The way I’ve always done it in the past is using list comprehension with something like:

r = re.compile('.*search.*')
if [line for line in output if r.match(line)]:
  do_stuff()

Which I now realize is pretty inefficient. If the very first item in the list matches, we can skip all the rest of the comparisons and move on. I could improve this with:

r = re.compile('.*search.*')
for line in output:
  if r.match(line):
    do_stuff()
    break

But I’m wondering if there’s a more pythonic way to do this.

Asked By: ewok

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Source

Answer 1

You can use the builtin any():

r = re.compile('.*search.*')
if any(r.match(line) for line in output):
    do_stuff()

Passing in the lazy generator to any() will allow it to exit on the first match without having to check any farther into the iterable.

Answered By: MrAlexBailey

Answer 2

Given that I am not allowed to comment yet, I wanted to provide a small correction to MrAlexBailey’s answer, and also answer nat5142’s question. Correct form would be:

r = re.compile('.*search.*')
if any(r.match(line) for line in output):
    do_stuff()

If you desire to find the matched string, you would do:

lines_to_log = [line for line in output if r.match(line)]

In addition, if you want to find all lines that match any compiled regular expression in a list of compiled regular expressions r=[r1,r2,…,rn], you can use:

lines_to_log = [line for line in output if any(reg_ex.match(line) for reg_ex in r)]

Answered By: I. Jovanov

Answer 3

Starting Python 3.8, and the introduction of assignment expressions (PEP 572) (:= operator), we can also capture a witness of an any expression when a match is found and directly use it:

# pattern = re.compile('.*search.*')
# items = ['hello', 'searched', 'world', 'still', 'searching']
if any((match := pattern.match(x)) for x in items):
  print(match.group(0))
# 'searched'

For each item, this:

Applies the regex search (pattern.match(x))
Assigns the result to a match variable (either None or a re.Match object)
Applies the truth value of match as part of the any expression (None -> False, Match -> True)
If match is None, then the any search loop continues
If match has captured a group, then we exit the any expression which is considered True and the match variable can be used within the condition’s body

Answered By: Xavier Guihot

Answer 4

In reply to a question asked by @nat5142, in the answer given by @MrAlexBailey:
“Any way to access the matched string using this method? I’d like to print it for logging purposes”, assuming “this” implies to:

if any(re.match(line) for line in output):
    do_stuff()

You can do a for loop over the generator

# r = re.compile('.*search.*')
for match in [line for line in output if r.match(line)]:
    do_stuff(match) # <- using the matched object here

Another approach is mapping each match with the map function:

# r = re.compile('.*search.*')
# log = lambda x: print(x)
map(log, [line for line in output if r.match(line)])

Although this does not involve the “any” function and might not even be close to what you desire…

I thought this answer was not very relevant so here’s my second attempt…
I suppose you could do this:

# def log_match(match):
#    if match: print(match)
#    return match  
if any(log_match(re.match(line)) for line in output):
    do_stuff()

Answered By: tinnick

If any strings in a list match regex

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