Question Re: Do regular expressions support backreferences? in Technology Q&A

Do regular expressions support backreferences?

VolkerBarth — Tue, 18 Sep 2018 06:56:05 GMT

When using groups within a regex, it is usually possible to "back reference" to such a group via its number (or name), such as "\\1" to reference to the first (capturing) group.

Am I right that this is not supported with SQL Anywhere?

Here's a sample:

-- search for a number followed by non-digits and a further number
select regexp_substr('abc-101-abd-102-abe', '(\\\\d+)\\\\D+\\\\d+');
-- -> returns "101-abd-102"

-- If I want the match to restrict to those cases
-- where the first and second number are identical,
-- that requires a backreference via \\1, such as
select regexp_substr('abc-101-abd-102-abe', '(\\\\d+)\\\\D+\\\\1'); -- should not match
select regexp_substr('abc-101-abd-101-abe', '(\\\\d+)\\\\D+\\\\1'); -- should match

However, adding '\\\\1' to the regexp pattern leads to SQLCODE -1135 aka "Invalid regular expression: unsupported syntax in '(\\d+)\\D+\\1'"

I'm using 16.0.0.2704 but I guess the regex support has been unchanged for quite a number of versions.

Re: Do regular expressions support backreferences?

VolkerBarth — Tue, 18 Sep 2018 07:34:25 GMT

If backreferences are not supported, that would explain the following doc quote IMHO:

(?: pattern ) Non-capturing block
This is functionally equivalent to just pattern, and is provided for compatibility.

If backreferences are not supported, a group needs no capturing, and so a "capturing group" and a "non-capturing group" would be equivalent...

Re: Do regular expressions support backreferences?

ian_mchardy — Tue, 18 Sep 2018 09:13:49 GMT

Hi Volker,

SQL Anywhere does NOT support back references in regular expressions.

See https://help.sap.com/viewer/40c01c3500744c85a02db71276495de5/17.0/en-US/81735cab6ce21014b8a287b4fbb8fdc0.html for the list of escaped characters supported, and note that back references are not listed.

Ian

Re: Do regular expressions support backreferences?

VolkerBarth — Tue, 18 Sep 2018 09:49:47 GMT

Thanks for the confirmation. I noticed that list but was not sure as the docs also state:

For REGEXP and REGEXP_SUBSTR, regular expression syntax and support is consistent with Perl 5.

which includes backreferences AFAIK.

Re: Do regular expressions support backreferences?

Breck_Carter — Wed, 19 Sep 2018 09:34:59 GMT

. . . cannot . . . resist . . . must . . . post . . . this 🙂

Re: Do regular expressions support backreferences?

VolkerBarth — Wed, 19 Sep 2018 10:02:51 GMT

Particularly funny as I came across that one by chance (?) yesterday, too:)