Find regex occurences of a set of patterns in correct order with Python

Question

I am parsing a series of text files for some patterns, since I want to extract them to other file.

A way to say it is that I would like to "remove" everything except the matches from the file.

For example, if I have pattern1, pattern2, pattern3 as matching patterns, I'd like the following input:

bla bla
pattern1
pattern2
bla bla bla
pattern1
pattern3
bla bla bla
pattern1

To give the following output:

pattern1
pattern2
pattern1
pattern3
pattern1

I can use re.findall and successfully get the list of matches for any pattern, but I cannot think of a way to KEEP THE ORDER considering the matches of each pattern are mixed inside the file.

Thanks for reading.

wrote a copy-and-go solution based on @richards solution

Inbar Rose
– Inbar Rose

2012-08-01 14:54:00 +00:00
Commented Aug 1, 2012 at 14:54 — Inbar Rose
– Inbar Rose, Commented Aug 1, 2012 at 14:54

Richard · Accepted Answer · 2012-08-01 14:35:32Z

5

Combine it all into a single pattern. With your example code, use the pattern:

^pattern[0-9]+

If it's actually more complex, then try

^(aaaaa|bbbbb|ccccc|ddddd)

answered Aug 1, 2012 at 14:35

Richard

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8 Comments

Inbar Rose Over a year ago

i dont think this works for OP, he has multiple matches in his REGEX that he looks for, "pattern1,pattern2,etc" are examples... see my answer.

heltonbiker Over a year ago

I'll accept this answer, that's what I wanted to do, just didn't know how or didn't remember how. The multiple patterns using | (OR) is the key to get in order, cause it says "give me any match of the following patterns", and the result will then come already in order.

Inbar Rose Over a year ago

oh - i see what you did there now, yes, the second "more complex" regex would work. but OP should still use file.writelines() itterating over the list that re.findall() returns.

heltonbiker Over a year ago

just for the record, my unholy pattern is: '<p class="docText">.+?</p>|<h3 class="docSection1Title">.+?</h3>' ;o)

Abe Karplus Over a year ago

@heltonbiker Have you not seen stackoverflow.com/questions/1732348/…? Do not try to parse HTML with regular expressions.

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Inbar Rose · Accepted Answer · 2012-08-01 14:51:01Z

2

here is an answer in "copy this and go" format.

import re

#lets you add more whenever you want
list_of_regex = [r"aaaa",r"bbbb",r"cccc"]

#hold the completed pattern
pattern_string = r"^("

#combines the patterns
for item in list_of_regex:
    pattern_string += "|".join(list_of_regex)

pattern_string += r")"

#open the file that you are reading
fr = open(FILE_TO_READ)

#holds the read files strings
search_string = fr.read()

#close the file
fr.close()

#open the file you want to write to
fw = open(FILE_TO_WRITE, 'w')

#write the results of findall into the file (as requested)
fw.writelines(re.findall(pattern_string,search_string))

#close the file
fw.close()

answered Aug 1, 2012 at 14:51

Inbar Rose

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2 Comments

Inbar Rose Over a year ago

based off of @richards answer

heltonbiker Over a year ago

I suspect re.compile could be of some help here, but I'd have to look more thoroughly. Thanks anyway!

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