SpaCy Matcher with optional suffix in pattern reports multiple matches on same text

Question

Using the following Matcher rule:

{'label': 'R-1',
 'pattern': [{'TEXT': 'MyLabel'}, {'TEXT': ':', 'OP': '?'}],
 'greedy': 'LONGEST', }

on the text: 'MyLabel: Some Value'

I get two matches: 'MyLabel' and 'MyLabel:'

For me, that was quite surprising - I was expecting a single match on 'MyLabel:'. Adding the new greedy flag didn't make any difference.

• Is this the intended behavior or is it a bug?
• How should I determine that the second match really is just a subset of the first match?
• Will the shorter match always be reported before the longer match?

SpaCy version 3.7.5

Answer 1

i will say that the behavior you're observing with the SpaCy Matcher is expected, and it is not a bug. When you use the {'TEXT': ':', 'OP': '?'} pattern, the OP: '?' operator means that the colon is optional, so the matcher will generate both the shorter and the longer match, as you've seen.

Explanation:

• Pattern: {'TEXT': 'MyLabel'}, {'TEXT': ':', 'OP': '?'}.
• Text: 'MyLabel: Some Value'.

So for this pattern, SpaCy will try to match:

1. 'MyLabel' alone (because the colon is optional).
2. 'MyLabel:' (because the colon can be included).

Therefore, you will get two matches: 'MyLabel' and 'MyLabel:'.

Now to Answer Your Questions:

1. Is this the intended behavior or is it a bug?

   • This is intended behavior. The OP: '?' operator allows the colon to be optionally matched, leading to multiple matches.

2. How should I determine that the second match really is just a subset of the first match?

   • To determine if one match is a subset of another, you can compare the start and end indices of the matches. The longer match will have the same start index but a different end index. Now i wrote a code below even using spacy version 3.7.5, see details below

pip show spacy
Name: spacy
Version: 3.7.5
Summary: Industrial-strength Natural Language Processing (NLP) in Python
Home-page: https://spacy.io
Author: Explosion
Author-email: [email protected]
License: MIT
Location: /home/adesoji/Downloads/visis-backend-assessment-Adesoji/visisenv/lib/python3.11/site-packages
Requires: catalogue, cymem, jinja2, langcodes, murmurhash, numpy, packaging, preshed, pydantic, requests, setuptools, spacy-legacy, spacy-loggers, srsly, thinc, tqdm, typer, wasabi, weasel
Required-by: en-core-web-sm

Now Example in code:

import spacy
from spacy.matcher import Matcher

nlp = spacy.load("en_core_web_sm")
doc = nlp("MyLabel: Some Value")

matcher = Matcher(nlp.vocab)
pattern = [{'TEXT': 'MyLabel'}, {'TEXT': ':', 'OP': '?'}]
matcher.add("R-1", [pattern])

matches = matcher(doc)
for match_id, start, end in matches:
    span = doc[start:end]
    print(f"Match: {span.text}, Start: {start}, End: {end}")

# Now, we Determine if one match is a subset of another
matches.sort(key=lambda x: (x[1], -x[2]))  # Sort by start index, then by end index descending
filtered_matches = []
last_end = -1
for match_id, start, end in matches:
    if start >= last_end:  # This is for Avoiding adding subsets
        filtered_matches.append((match_id, start, end))
        last_end = end

for match_id, start, end in filtered_matches:
    span = doc[start:end]
    print(f"Filtered Match: {span.text}")

Now, This code will filter out the shorter match and your output will be

Match: MyLabel, Start: 0, End: 1
Match: MyLabel:, Start: 0, End: 2
Filtered Match: MyLabel:   , you can see MYLabel: with the colon symbol there

3. Now Will the shorter match always be reported before the longer match?
   • I don't think the matches are not guaranteed to be reported in a specific order. so to handle this, you can sort the matches by their start and end indices as shown in the code example above.Now, After sorting, you can now filter out matches that are subsets of longer matches.

Another Alternative Solution:

If you want to ensure that only the longest match is returned, you can change the way you define the pattern:

pattern = [{'TEXT': 'MyLabel'}, {'TEXT': ':', 'OP': '?', 'greedy': 'LONGEST'}]

note that the greedy flag doesn't change the behavior of matching itself but rather can influence how overlaps are handled in certain custom settings.

Now back to the Summary of what i explained:

• The behavior you're seeing is by design, due to the optional OP: '?' operator.
• in addition, you can filter out the shorter match by comparing start and end indices of the matches.
• furthermore, Sorting the matches by start and end indices allows you to keep only the longest, non-overlapping matches.