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

You don't anchor your regex, as has been said. Another problem is that \w also matches digits... Now look at how the regex engine proceeds to match with your input:

# begin
regex: |(?=(\d+))\w+\1
input: |456x56
# lookahead (first group = '456')
regex: (?=(\d+))|\w+\1
input: |456x56 
# \w+
regex: (?=(\d+))\w+|\1
input: 456x56|
# \1 cannot be satisfied: backtrack on \w+
regex: (?=(\d+))\w+|\1
input: 456x5|6 
# And again, and again... Until the beginning of the input: \1 cannot match
# Regex engine therefore decides to start from the next character:
regex: |(?=(\d+))\w+\1
input: 4|56x56
# lookahead (first group = '56')
regex: (?=(\d+))|\w+\1
input: 4|56x56
# \w+
regex: (?=(\d+))\w+|\1
input: 456x56|
# \1 cannot be satisfied: backtrack
regex: (?=(\d+))\w+|\1
input: 456x5|6
# \1 cannot be satisfied: backtrack
regex: (?=(\d+))\w+|\1
input: 456x|56
# \1 satified: match
regex: (?=(\d+))\w+\1|
input: 4<56x56>