perl - perl split() 中的分隔符保留模式

Question

split /PATTERN/,EXPR

我在书中读到以下内容，

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

我似乎找不到详细解释这一点的文档。有人可以简要解释一下分隔符保留模式及其可能的用法吗？

Answer 1

这记录在perldoc -f split中接近尾声(代码中的注释是我自己的):

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 (!)

<小时/>

因此，有两个有趣的怪癖可能会引起粗心的人的注意:

• 只要捕获组不匹配，但 PATTERN 中的其他内容匹配，就会在列表上下文中生成 undef

• 您可以使用捕获组进行拆分，并将 LIMIT 指定为 $n，结果列表的内容会超过 $n元素