如何匹配字符串中特定范围之间的特定字符？

Question

I have this string:

$str = "here is start of rage, also here is some text and here is the end of string";
//              ^^^^^                                                 ^^^
//                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Now I'm trying to remove all e letter between this range: [start - end]. Well I want this output:

$newstr = "here is start of rag, also hr is som txt and hr is th end of string";

How can I do that?

Answer 1

With preg_replace and the \G anchor:

echo preg_replace('~(?:\G(?!\A)|\bstart\b)[^e]*\K(?:\Be|e(?!nd\b))~S', '', $str);

details:

~
(?:
    \G(?!\A)   # contiguous to the previous match, not at the start of the string
  |            # OR
    \bstart\b  # the word "start"
)
[^e]*          # all that is not an "e"
\K             # exclude all previous matched characters from the whole match 
(?:
    \Be        # an "e" that is not the start of a word
  |            # OR
    e(?!nd\b)  # an "e" that is not followed by "nd"
)
~  
S  # the STUDY modifier that tries to improve non-anchored patterns

This pattern find one "e" at a time. Once the word "start" is found, the \G anchor forces the next match to be contiguous, since it matches the position at the end of the previous match. When the word "end" is reached (?:\Be|e(?!nd\b)) fails and the contiguity is broken (until an other eventual word "start").

Note that this pattern doesn't check if the word "end" exists in the string (but it can easily be done). If the word "end" doesn't exist, all "e" will be removed from the word "start" until the end of the string.