Standard Meta Characters
Backreferences (continued)
Within the same regular expression, use a backslash followed by an integer as a backreference. The integer corresponding to a given pair of parentheses is determined by counting left parentheses from the beginning of the pattern. The following regular expression matches something similar to an HTML tag (like text here):
| /<(.*)>.*<\/\1>/ |
The following regular expression matches a string (at least four-characters long) whose first two characters are also its last two characters, but in reverse order (such as "abcdefba"):
| /^(.)(.).*\2\1$/ |