regex - how to capture pattern without different pattern before it?

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I'm trying to parse out prices but ignore two patterns that are also prices. One of the exclusions is the total price which is at the end which I am using lookahead to ignore. The second exclusion is if there's a variation of the letter Q before a price, for example Q10.00 or Q AWSMSN11.32 but I want to include if there's a three letter alpha that happens to end in Q such as YMQ234.03.

I've added a negative lookbehind but can't seem to get what I want.

This is the pattern I've tried: (?<![Q\d]) ?M?(\d+\.\d{2})(?=.*\d+\.\d{2}END)

test strings

ABC WS YMQ234.03WS TOY234.03USD468.06END
FUR BB LAB Q10.00 199.00USD209.00END
YAS DG TYY Q AWSMSN11.32 2503.08LD VET Q JKLOLE11.32 2503.08USD5028.80END
PPP VP LAP Q10.00 M342.41EE SFD Q10.00 282.24USD644.65END

regex101

Expected output

+---------------------------------------------------------------------------+---------+---------+
| ABC WS YMQ234.03WS TOY234.03USD468.06END                                  | 234.03  | 234.03  |
| FUR BB LAB Q10.00 199.00USD209.00END                                      | 199.00  |         |
| YAS DG TYY Q AWSMSN11.32 2503.08LD VET Q JKLOLE11.32 2503.08USD5028.80END | 2503.08 | 2503.08 |
| PPP VP LAP Q10.00 M342.41EE SFD Q10.00 282.24USD644.65END                 | 342.41  | 282.24  |
+---------------------------------------------------------------------------+---------+---------+
python regex
4 Answers

You might also match what you don't want, and capture what you do want.

Match optional whitespace and uppercase chars where there is a Q and match the decimal value that follows.

Make the exception of eliminating this match asserting that it is not preceded by 2 times an uppercase A-Z followed by Q

After the alternation, capture the decimal value in group 1, asserting that it is not followed by END

\b[A-Z ]*Q[A-Z ]*(?<![A-Z][A-Z]Q)\d+\.\d+|(\d+\.\d{2})(?!END)

Explanation

  • \b[A-Z ]*Q[A-Z ]* Word boundary, match a Q between optional spaces and uppercase chars
  • (?<![A-Z][A-Z]Q) Negative lookbehind, assert not 2 uppercase chars A-Z followed by Q directly to the left
  • \d+\.\d+ Match a decimal value
  • | Or
  • ( Capture group 1
    • \d+\.\d{2} Match 1+ digits followed by a dot and 2 digits
  • ) Close group 1
  • (?!END) Negative lookahead, assert what is directly to the right is not END

Regex demo | Python demo

For example

import re

regex = r"\b[A-Z ]*Q[A-Z ]*(?<![A-Z][A-Z]Q)\d+\.\d+|(\d+\.\d{2})(?!END)"
strings = [
    "ABC WS YMQ234.03WS TOY234.03USD468.06END",
    "FUR BB LAB Q10.00 199.00USD209.00END",
    "YAS DG TYY Q AWSMSN11.32 2503.08LD VET Q JKLOLE11.32 2503.08USD5028.80END",
    "PPP VP LAP Q10.00 M342.41EE SFD Q10.00 282.24USD644.65END"
]

for str in strings:
    print('{}: {}'.format(str, [x.group(1) for x in re.finditer(regex, str) if x.group(1)]))

Output

ABC WS YMQ234.03WS TOY234.03USD468.06END: ['234.03', '234.03']
FUR BB LAB Q10.00 199.00USD209.00END: ['199.00']
YAS DG TYY Q AWSMSN11.32 2503.08LD VET Q JKLOLE11.32 2503.08USD5028.80END: ['2503.08', '2503.08']
PPP VP LAP Q10.00 M342.41EE SFD Q10.00 282.24USD644.65END: ['342.41', '282.24']
user_54249880

By matching the following regular expression the values of interest will be saved to capture group 1.

r'(?=[^Q\d]*(?=\d))(?:(?<!Q)|(?<=[A-Z]{2}Q)|\D*\d+\.\d{2})[^Q\d]*([1-9]\d*\.\d{2})(?!END\b)'

Start your engine! | Python code

Python's regex engine performs the following operations.

(?=               : begin positive lookahead
  [^Q\d]*         : match 0+ chars other that 'Q' and digits
  (?=\d)          : positive lookahead asserts next char is a digit
)                 : end positive lookahead
(?:               : begin non-capture group
  (?<!Q)          : negative lookbehind asserts current match did
                    not end with 'Q'
|                 : or
  (?<=            : begin positive lookbehind
    [A-Z]{2}Q     : match two letters, 'Q'
  )               : end positive lookbehind
|                 : or
  \D*\d+\.\d{2}   : match 0+ non-digits, 1+ digits, '.', 2 digits
)                 : end non-capture group
[^Q\d]*           : match 0+ chars other than 'Q' or digits
([1-9]\d*\.\d{2}) : match digit other than zero, 0+ digits, '.',
                    2 digits
(?!END\b)         : negative lookahead asserts current match is
                  : not followed by 'END'

The positive lookahead (?=[^Q\d]*(?=\d)) leaves the regex engine's internal string pointer at its current location if 'Q' does not appear between the current location and the first digit of a string that matches [1-9]\d*\.\d{2}. Otherwise it moves the pointer to just after the last 'Q' that precedes the first digit of a string that matches [1-9]\d*\.\d{2}.

user_2569700

You could use regex module instead of re with the pattern:

Q[A-Z ]*(?<!\b[A-Z]{2}Q)[\d.]+(*SKIP)(*F)|\d+(?:\.\d+)(?!\d*END$)

See the online demo.

In Python this could look like:

import regex
arr = ['ABC WS YMQ234.03WS TOY234.03USD468.06END', 'FUR BB LAB Q10.00 199.00USD209.00END', 'YAS DG TYY Q AWSMSN11.32 2503.08LD VET Q JKLOLE11.32 2503.08USD5028.80END', 'PPP VP LAP Q10.00 M342.41EE SFD Q10.00 282.24USD644.65END']
res = [regex.findall(r'Q[A-Z ]*(?<!\b[A-Z]{2}Q)[\d.]+(*SKIP)(*F)|\d+(?:\.\d+)(?!\d*END$)',x) for x in arr]
print(res)

Prints:

[['234.03', '234.03'], ['199.00'], ['2503.08', '2503.08'], ['342.41', '282.24']]
user_97581940

You may use a simple well-known trick when you need to discard some matches: use an optional capturing group that will only match when a failure is expected:

(\bQ\s?[A-Z]*)?(?<!\d)(\d+\.\d{2})(?=.*\d\.\d{2}END)
|_____________|

See the regex demo. Whenever match data object Group 1 is not empty, the match should be dropped.

Regex details

  • (\bQ\s?[A-Z]*)? - an optional capturing group #1 that captures
    • \bQ - a word boundary followed with Q
    • \s? - one or zero whitespaces
    • [A-Z]* - any 0 or more ASCII uppercase letters
  • (?<!\d) - no digit immediately on the left is allowed
  • (\d+\.\d{2}) - 1+ digits, . and then any two digits
  • (?=.*\d\.\d{2}END) - any 0 or more chars other than line break chars as many as possible followed with a digit, ., two digits and END must appear immediately to the right of the current location.

See the Python implementation with re:

import re
strings = ['ABC WS YMQ234.03WS TOY234.03USD468.06END','FUR BB LAB Q10.00 199.00USD209.00END','YAS DG TYY Q AWSMSN11.32 2503.08LD VET Q JKLOLE11.32 2503.08USD5028.80END','PPP VP LAP Q10.00 M342.41EE SFD Q10.00 282.24USD644.65END']
rx = r'(\bQ\s?[A-Z]*)?(?<!\d)(\d+\.\d{2})(?=.*\d\.\d{2}END)'
for s in strings:
    matches = [x.group(2) for x in re.finditer(rx, s) if not x.group(1)] # note the if condition that drops unwlecome matches
    print(s, matches, sep=" => ")

Output:

ABC WS YMQ234.03WS TOY234.03USD468.06END => ['234.03', '234.03']
FUR BB LAB Q10.00 199.00USD209.00END => ['199.00']
YAS DG TYY Q AWSMSN11.32 2503.08LD VET Q JKLOLE11.32 2503.08USD5028.80END => ['2503.08', '2503.08']
PPP VP LAP Q10.00 M342.41EE SFD Q10.00 282.24USD644.65END => ['342.41', '282.24']
user_38329700
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