Regexp - Assertion (Condition)
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.
|\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).