Regex Statement to only match parts of a string for comparison - Python

What I am trying to do is match values from one file to another, but I only need to match the first portion of the string and the last portion.

I am reading each file into a list, and manipulating these based on different Regex patterns I have created. Everything works, except when it comes to these type of values:

V-1\ZDS\R\EMBO-20-1:24
V-1\ZDS\R\EMBO-20-6:24

In this example, I only want to match 'V-1\ZDS\R\EMBO-20' and then compare the '24' value at the end of the string. The number x in '20-x:', can vary and doesn't matter in terms of comparisons, as long as the first and last parts of this string match.

This is the Regex I am using:

re.compile(r"(?:.*V-1\\ZDS\\R\\EMBO-20-\d.*)(:\d*\w.*)")

Once I filter down the list, I use the following function to return the difference between the two sets:

funcDiff = lambda x, y: list((set(x)- set(y))) + list((set(y)- set(x)))

Is there a way to take the list of differences and filter out the ones that have matching values after the

: 

as mentioned above?

I apologize is this is an obvious answer, I'm new to Python and Regex!

The output I get is the differences between the entire strings, so even if the first and last part of the string match, if the number following the 'EMBO-20-x' doesn't also match, it returns it as being different.

Before discussing your question, regex101 is an incredibly useful tool for this type of thing.

Your issue stems from two issues:

1.) The way you used .*

2.) Greedy vs. Nongreedy matches

.* kinda sucks

.* is a regex expression that is very rarely what you actually want.
> As a quick aside, a useful regex expression is [^c]* or [^c]+. These expressions match any character except the letter c, with the first expression matching 0 or more, and the second matched 1 or more.

.* will match all characters as many times as it can. Instead, try to start your regex patterns with more concrete starting points. Two good ways to do this are lookbehind expressions and anchors.

> Another quick aside, it's likely that you are misusing regex.match and regex.find. match will only return a match that begins at the start of the string, while find will return matches anywhere in the input string. This could be the reason you included the .* in the first place, to allow a .match call to return a match deeper in the string.

Lookbehind Expressions

There are more complete explanations online, but in short, regex patterns like:

(?<=test)foo

will match the text foo, but only if test is right in front of it. To be more clear, the following strings will not match that regex:

foo
test-foo
test foo

but the following string will match:

testfoo

This will only match the text foo, though.

Anchors

Another option is anchors. ^ and $ are special characters, matching the start and end of a line of text. If you know your regex pattern will match exactly one line of text, start it with ^ and end it with $.

Leading patterns with .* and ending with .* are likely the source of your issue. Although you did not include full examples of your input or your code, you likely used match as opposed to find.

In regex, . matches any character, and * means 0 or more times. This means that for any input, your pattern will match the entire string.

Greedy vs. Non-Greedy qualifiers

The second issue is related to greediness. When your regex patterns have a * in them, they can match 0 or more characters. This can hide problems, as entire * expressions can be skipped. Your regex is likely matched several lines of text as one match, and hiding multiple records in a single .*.

The Actual Answer

Taking all of this in to consideration, let's assume that your input data looks like this:

V-1\ZDS\R\EMBO-20-1:24
V-1\ZDS\R\EMBO-20-6:24
V-1\ZDS\R\EMBO-20-3:93
V-1\ZDS\R\EMBO-20-6:22309
V-1\ZDS\R\EMBO-20-8:2238
V-1\ZDS\R\EMBO-20-3:28

A better regular expression would be:

^V-1\\ZDS\\R\\EMBO-20-\d:(\d+)$

To visualize this regex in action, follow this link.

There are several differences I would like to highlight:

• Starting the expression with ^ and ending with $. This forces the regex to match exactly one line. Even though the pattern works without these characters, it's good practice when working with regex to be as explicit as possible.

• No useless non-capturing group. Your example had a (?:) group at the start. This denotes a group that does not capture it's match. It's useful if you want to match a subpattern multiple times ((?:ab){5} matches ababababab without capturing anything). However, in your example, it did nothing

• Only capturing the number. This makes it easier to extract the value of the capture groups.

• No use of *, one use of +. + works like *, but it matches 1 or more. This is often more correct, as it prevents 'skipping' entire characters.