The above points have taken up a lot of space. Now we will introduce some concepts of boundary matching characters and groups.
The general boundary matching characters are as follows:
| Syntax | Description |
| ^ | Matches the beginning of the string (matches the beginning of each line in the case of multiple lines) |
| $ | Matches the end of the string (matches the end of each line in the case of multiple lines) |
| \A | Matches only The beginning of the string |
| \Z | matches only the end of the string |
| \b | matches \w and \W |
| \B | [^\b] |
grouping, expression enclosed in parentheses The formula is grouping. The grouping expression (...) actually treats this part of characters as a whole. Of course, there can be multiple groups. Every time a group is encountered, the number will be increased by 1, and quantifiers can also be added after the grouping.
There should be examples here, but considering the space issue, I won’t post them
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