|Group 1||Group 2||Group 3||Group 4|
An expression can ask to match the exact characters
that a capture group has captured.
In an expression,
\1 means whatever
characters are currently in group number 1.
first matches ant or bat or cat, matches a space, and then matches whichever of the three animals started the string.
It matches ant ant or bat bat or cat cat but not ant bat nor similar strings.
\1 is called a back reference
or sometimes called a variable.
Other variables are
\3 , and so on.
(\w+) (\w+) \w+ \2 \1
fall leaves after leaves fall
Write an expression that matches strings that have an 'X' in the middle and an equal number of 'a' on either side of the 'X'.
For example, aaaaXaaaa and X and aaaaaaaXaaaaaaa are strings that match.