Looking for a regular expression for the following pattern.

skooma has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Looking for a regular expression for the following pattern. by ikegami (Patriarch) on Mar 29, 2018 at 12:41 UTC
`[0-9_][a-zA-Z][a-zA-Z0-9_]` [download] So to check if an entire string matches one of your sequences, `if ($str =~ /^[0-9_][a-zA-Z][a-zA-Z0-9_]\z/) { ... }` [download]	[reply] [d/l] [select]
Re^2: Looking for a regular expression for the following pattern. by Skeeve (Parson) on Mar 29, 2018 at 17:35 UTC
++ for finally making me clear what's \z is good for… `s$$([},&%#}/&/]+}%&{});#$&&s&&$^X.($'^"%]=\&(\|?{%` `+`.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e	[reply] [d/l] [select]
Re: Looking for a regular expression for the following pattern. by Skeeve (Parson) on Mar 29, 2018 at 06:37 UTC
So is it alway an underscore after 2 characters? Otherwise this should do: `/^[a-z0-9_]+$/i` [download] `s$$([},&%#}/&/]+}%&{});#$&&s&&$^X.($'^"%]=\&(\|?{%` `+`.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e	[reply] [d/l] [select]
Re^2: Looking for a regular expression for the following pattern. by ikegami (Patriarch) on Mar 29, 2018 at 12:37 UTC
That matches "_" and "a\n"	[reply]
Re^3: Looking for a regular expression for the following pattern. by Skeeve (Parson) on Mar 29, 2018 at 14:06 UTC
OP didn't say anything about linefeeds so it's safe to assume they can be ignored OP didn't say that "_" is not allowed. It's "alphanumeric characters with underscores, one or more" OP's post is poorly written so expect a poorly written reply ;) `s$$([},&%#}/&/]+}%&{});#$&&s&&$^X.($'^"%]=\&(\|?{%` `+`.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e	[reply] [d/l] [select]
Re^4: Looking for a regular expression for the following pattern. by Laurent_R (Canon) on Mar 29, 2018 at 16:03 UTC
Re: Looking for a regular expression for the following pattern. by Marshall (Canon) on Mar 29, 2018 at 09:57 UTC
I don't understand your question. Please give more context information about what to match or not match. In Perl regex lingo \w+ matches `a-zA-Z0-9_` Give an example of input data and what you expect to match. Update: I looked back at this thread, but I have yet to see a clear problem statement. Show an example of the "real world" data and what you expect to match from that. Writing regex'es out of context just doesn't make any sense.	[reply] [d/l]
Re: Looking for a regular expression for the following pattern. by LanX (Saint) on Mar 29, 2018 at 10:28 UTC
How (Not) To Ask A Question Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Wikisyntax for the Monastery}	[reply]
Re: Looking for a regular expression for the following pattern. by hdb (Monsignor) on Mar 29, 2018 at 07:54 UTC
Groups of at least two alphanumeric characters separated by underscores: `/^(([a-zA-Z0-9]+)_)*[a-zA-Z0-9]+$/` [download]	[reply] [d/l]
Re^2: Looking for a regular expression for the following pattern. by ikegami (Patriarch) on Mar 29, 2018 at 12:39 UTC
That matches "1" and "a\n". It doesn't match "a_".	[reply]
Re^3: Looking for a regular expression for the following pattern. by hdb (Monsignor) on Mar 29, 2018 at 13:33 UTC
Next try... `/^(([a-zA-Z0-9]{2,}_)*[a-zA-Z0-9]{2,})$/ && print "$_\n" for "1", "a\n", "aa\n", "a_", "aa", "aa_bb_cc_dd", "a1_11_1c_11", +"aaa", "aa_";` [download] Still matches a trailing newline. It does not match "aa_" on purpose as this is not a sequence of two or more alphanumeric characters separated by underscores which was my claim if not the original question.	[reply] [d/l]
Re: Looking for a regular expression for the following pattern. by AnomalousMonk (Archbishop) on Mar 29, 2018 at 14:33 UTC
I, also, don't really understand the ~~requirements~~ \| vaguely-stated requirements of the OP. The way I'd like to see a question like this posted (and a way that'd be more likely to get useful responses IMHO) is something like: Read more... (2 kB) Of course, one would omit the test regex or use a dummy in an initial query post. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: Looking for a regular expression for the following pattern. by Anonymous Monk on Mar 29, 2018 at 14:48 UTC
Well, if the three alternatives are a bit tricky to reconcile, why not simply use three regexes and perhaps an `if` statement?	[reply]
Re^2: Looking for a regular expression for the following pattern. by AnomalousMonk (Archbishop) on Mar 29, 2018 at 14:58 UTC
No number of regexes and/or `if`-clauses will reconcile a requirement statement that doesn't state what is required. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: Looking for a regular expression for the following pattern. by Anonymous Monk on Mar 30, 2018 at 02:22 UTC
The first two requirements sound like the same thing: `/([:alnum:]+[_]+/`. But the third requirement is a subset of the first, consuming the alnum sequence with no need for underscore. Two regexes joined by `\|\|` logical-or would do it. However, it just might be as simple as replacing the last `+` in the above regex with `*` to indicate "zero or more" underscores. The pattern now looks for one-or-more alnum characters followed by zero-or-more underscores.	[reply]
Re^4: Looking for a regular expression for the following pattern. by AnomalousMonk (Archbishop) on Mar 30, 2018 at 15:03 UTC