Fix regex parsing for parentheses

[nicolascanceill-Optiver]

This fixes the check for values' regexes to be enclosed within parentheses

(I am contributing this patch as an employee of Optiver, ready to sign a CLA)

Issue

• Define a value with the regex (boo\)hoo)
• Both (boo\)hoo) and boo\)hoo are valid regexes, so this should work
• However, this raises a TextFSMTemplateError instead

Background

Each value definition requires a valid regex which, as mentioned in the wiki, must be enclosed within parentheses. It is easy to check whether the regex starts with ( and ends with a non-escaped ) but the hard part is making sure that the ending ) matches the starting (.

This used to be enforced by:

• Making sure the regex starts with ( and ends with )
• Making sure the regex contains as many ( as )

This was not a correct check, so google#68 changed it to:

• Making sure the regex starts with ( and ends with )
• Making sure the regex contains as many ( as ) while ignoring any portion of the regex enclosed within non-escaped brackets

This is still not a correct check:

• It still fails on (boo\)hoo)
• It is implemented incorrectly, so it also fails on ([boo(hoo)])

Proposed fix

• The problem of enforcing that the regex starts with a ( and ends with a matching, non-escaped ) is non-trivial
• This can easily be offloaded to the re library by compiling the regex

[nicolascanceill-Optiver]

The CLA is now signed but I cannot re-run the check.

Trying to force a re-run by closing and reopening the PR.

[harro]

Thanks for the fix.