An assertion is an assertion that specifies a condition that has to be met at a particular point in a match, without consuming any characters from the subject string.
simple assertion designed by the backslash character.
| assertions | Description |
|---|---|
| \b | word boundary |
| \B | not a word boundary |
| \A | start of subject (independent of multiline mode) |
| \Z | end of subject or newline at end (independent of multiline mode) |
| \z | end of subject (independent of multiline mode) |
| \G | first matching position in subject |
These assertions may not appear in character classes (but note that “\b” has a different meaning, namely the backspace character, inside a character class).
Regexp - Look-around group (Assertion) - ( Lookahead | Lookbehind )