HPCC-31739 Document changes in moving Unicode regex from ICU to PCRE2

[JamesDeFabia]

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[github-actions]

Jira Issue: https://hpccsystems.atlassian.net//browse/HPCC-31739

Jirabot Action Result:
Workflow Transition: Merge Pending
Updated PR

[dcamper]

We have settled on using only one third-party library for regex support (PCRE2), no matter what data type is used. PCRE2 always supports the syntax outlined in pcre2pattern.html, even for Unicode (UNICODE or UTF8). When searching Unicode data, PCRE2 also supports the syntax outlined in pcre2unicode.html.

To avoid confusion, the description here might want to lead with that.

Background info: Reading what I just wrote, it may seem that the latter is a superset of the former. That is not the case. It all comes down to things like "what is a letter" and "what is a letter" and what character set is supported. Example: [[:digit:]] is a pattern that defines the numeric characters 0-9 in ASCII (our STRING datatype), but Unicode contains many different sets of numeric digits. [[:digit:]] will match that same set of 0-9 characters, even in UNICODE or UTF8, but it will not match the other numeric characters; to do that you need to use \p{Nd} instead (which will get them all, even the [[:digit:]] set).

The above applies to all three REGEX ECL functions.

Can the description be reworded?

[dcamper]

Looks good.

[JamesDeFabia]

@ghalliday This should be ready to merge