Separator Retention Mode in perl split()

Question

split /PATTERN/,EXPR

I read the following in a book,

When you use a pattern in split, be sure to avoid memory parantheses in the pattern since these trigger seperator retention mode.

I can't seem to find the documentation which explains this in detail. Could someone please explain Seperator Retention Mode and its possible usage briefly?

Answer 1

This is documented in perldoc -f split towards the end (in-code commentary is my own):

If the PATTERN contains capturing groups, then for each separator, an additional field is produced for each substring captured by a group (in the order in which the groups are specified, as per backreferences); if any group does not match, then it captures the undef value instead of a substring. Also, note that any such additional field is produced whenever there is a separator (that is, whenever a split occurs), and such an additional field does not count towards the LIMIT. Consider the following expressions evaluated in list context (each returned list is provided in the associated comment):

split(/-|,/, "1-10,20", 3)       # ('1', '10', '20')
                                 # No retention, '-', ',' consumed

split(/(-|,)/, "1-10,20", 3)     # ('1', '-', '10', ',', '20')
                                 # Split on and retain '-' or ','
                                 # 5 elements returned

split(/-|(,)/, "1-10,20", 3)     # ('1', undef, '10', ',', '20')
                                 # undef because '-' matches

split(/(-)|,/, "1-10,20", 3)     # ('1', '-', '10', undef, '20')
                                 # undef because ',' matches

split(/(-)|(,)/, "1-10,20", 3)   # ('1', '-', undef, '10', undef, ',', '20')
                                 # one match per capturing group. (-) matches -, but
                                 # (,) returns undef on trying to match -.
                                 # 7 elements (!)

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So, two interesting quirks that may catch out the unwary:

• The generation of undefs in list context whenever a capturing group does not match, but something else in PATTERN does

• You might split with a capture group, specifying LIMIT as $n, and the resultant list has more than $n elements

Answer 2

It means that if you use a regex with parentheses that generates back references, then the matched separators will be retained, and returned in the list, along with the split values.