Regexp matching a string - positive lookahead

Question

Regexp: (?=(\d+))\w+\1 String: 456x56

Hi,

I am not getting the concept, how this regex matches "56x56" in the string "456x56".

1. The lookaround, (?=(\d+)), captures 456 and put into \1, for (\d+)
2. The wordcharacter, \w+, matches the whole string("456x56")
3. \1, which is 456, should be followed by \w+
4. After backtracking the string, it should not find a match, as there is no "456" preceded by a word character

However the regexp matches 56x56.

Answer 1

5) Regex engines concludes that it cannot find a match if it start searching from 4, so it skips one character and searches again. This time, it captures two digits into \1 and ends up matching 56x56

If you want to match only whole strings, use ^(?=(\d+))\w+\1$

^ matches beginning of string
$ matches end of string