Zerosquare (./18) :
Et dans l'autre sens (regexp -> description) :
http://rick.measham.id.au/paste/explain.pl

regex101 l'explique aussi

/^(\s*)((\S+)\s+(\d+))?\s*#?(.*)/
^ assert position at start of the string
1st Capturing group (\s*)
\s* match any white space character [\r\n\t\f ]
Quantifier: * Between zero and unlimited times, as many times as possible, giving back as needed [greedy]
2nd Capturing group ((\S+)\s+(\d+))?
Quantifier: ? Between zero and one time, as many times as possible, giving back as needed [greedy]
Note: A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
3rd Capturing group (\S+)
\S+ match any non-white space character [^\r\n\t\f ]
Quantifier: + Between one and unlimited times, as many times as possible, giving back as needed [greedy]
\s+ match any white space character [\r\n\t\f ]
Quantifier: + Between one and unlimited times, as many times as possible, giving back as needed [greedy]
4th Capturing group (\d+)
\d+ match a digit [0-9]
Quantifier: + Between one and unlimited times, as many times as possible, giving back as needed [greedy]
\s* match any white space character [\r\n\t\f ]
Quantifier: * Between zero and unlimited times, as many times as possible, giving back as needed [greedy]
#? matches the character # literally
Quantifier: ? Between zero and one time, as many times as possible, giving back as needed [greedy]
5th Capturing group (.*)
.* matches any character (except newline)
Quantifier: * Between zero and unlimited times, as many times as possible, giving back as needed [greedy]