[New-bugs-announce] [issue32198] \b reports false-positives in Indic strings involving combining marks

[Shriramana Sharma]

New submission from Shriramana Sharma <samjnaa at gmail.com>:

Code:

import re
cons_taml = "[கஙசஞடணதநபமயரலவழளறன]"
print(re.findall("\\b" + cons_taml + "ை|ஐ", "ஐவர் பையன் இசை சிவிகை இல்லை இவ்ஐ"))
cons_deva = "[कखगघङचछजझञटठडढणतथदधनपफबभमयरलवशषसह]"
print(re.findall("\\b" + cons_deva + "ै|ऐ", "ऐषमः तैलम् ईडै समीशै ईक्षै ईक्ऐ"))

Specs:
Kubuntu Xenial 64 bit
Python 3.5.2 (default, Nov 23 2017, 16:37:01) 
[GCC 5.4.0 20160609] on linux

Actual Output:
['ஐ', 'பை', 'கை', 'லை', 'ஐ']
['ऐ', 'तै', 'शै', 'षै', 'ऐ']

Expected Output:
['ஐ', 'பை']
['ऐ', 'तै']

Rationale:

The formulated RE desires to identify words *starting* with the vowel /ai/ (\u0BC8 ை in Tamil script and \u0948 ै in Devanagari as vowel sign or \u0B90 ஐ \u0910 ऐ as independent vowel). ஐவர் பையன் and ऐषमः तैलम् are the only words fitting this criterion. \b is defined to mark a word boundary and is here applied at the beginning of the RE.

Observation:

There seems to be some assumption that only GC=Lo characters constitute words. Hence the false positives at ச ி வ ி (க ை) and स म ी (श ै) where the ி and ी are vowel signs, and இ ல ் (ல ை) and ई क ् (ष ै) where the ் and ् are virama characters or vowel cancelling signs.

In Indic, such GC=Mc and GC=Mn characters are inalienable parts of words. They should be properly identified as parts of words and no word boundary answering to \b should be generated at their positions.

----------
components: Regular Expressions
messages: 307430
nosy: ezio.melotti, jamadagni, mrabarnett
priority: normal
severity: normal
status: open
title: \b reports false-positives in Indic strings involving combining marks
type: behavior
versions: Python 3.5

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32198>
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