php PCRE Regex优化

Question

quite new to regexes i'm trying to optimize one, or at least know if there are better ways to do it.

Here is my input string:

$str = 'Some text
spanned on
several lines
txt_to_grab1 fixed_text1 txt_to_grab2
Full line to grab
txt_to_grab3 fixed_text2 txt_to_grab4
Some text after';

I'm trying to grab the lines from "txt_to_grab1" to "txt_to_grab4", but only the words "txt_to_grabX" and the line "Full line to grab".
I want to preserve everything untouched before and after (ie line breaks), but remove line breaks inside the lines i grab (as each line will be a <tr> that'll go into an html table).

Regex patterns/replace i found matching:

$find = "#(?<=
)(.*?) fixed_text1 (.*?)(
.*?
)(.*?) fixed_text2 (.*?)(
)#i";
$replace = '"$1" && "$2" grabbed.$3"$4" && "$5" grabbed.$6';   

$find = "#(.*)(?<=
)(.*?) fixed_text1 (.*?)(
)(.*)(?<=
)(.*?) fixed_text2 (.*?)(
.*)#is";
$replace = '$1"$2" && "$3" grabbed.$4$5"$6" && "$7" grabbed.$8';

Questions :

All questions can be sum up as : are there better/shorter/faster patterns ?

• how to make the patterns work with either or ? I read somewhere on stack that (? ) would be a solution, but i dunno how to use them in lookbehinds. For example the following patterns work, but i don't like them (dirty as only are used in lookbehinds, may produce unexpected results):

  "#(?<=
  )(.*?) fixed_text1 (.*?)(?
  .*??
  )(.*?) fixed_text2 (.*?)(?
  )#i"
  "#(.*)(?<=
  )(.*?) fixed_text1 (.*?)(?
  )(.*)(?<=
  )(.*?) fixed_text2 (.*?)(?
  .*)#is";
  

• even better, how to use the "s" modifier to remove all line breaks from the pattern, so being able to use (.*?) but still grabbing what i want ? Word boundaries ?

• is the multiline mode (m modifier) useful/helpful here ?

I'd really like the regexes to be explained, if you provide some :)

Answer 1

You don't need lookbehinds for this. Just use the start-of-line anchor at the beginning of your regex and the end-of-line anchor at the end (that's ^ and $ in multiline mode). To match the line separators in the middle you can use (?: |[ ]), a common idiom for the three most common styles of line separator: , , or .

As for the s modifier (a.k.a. "single-line" or "DOT_ALL"), you don't need that either. All it does is allow the dot metacharacter to match line separators as well as all other characters, which doesn't do you any good. You want it to stop matching when it reaches line breaks, so you can exclude them from your captures.

Here's a demo:

$pattern='#^(.*?) fixed_text1 (.*)(?:
|[
])(.*)(?:
|[
])(.*?) fixed_text2 (.*)$#im';

preg_match($pattern, $source, $m);

echo "$m[1] && $m[2] grabbed.
";
echo "$m[3]
";
echo "$m[4] && $m[5] grabbed.
"; 

output:

txt_to_grab1 && txt_to_grab2 grabbed.
Full line to grab
txt_to_grab3 && txt_to_grab4 grabbed.

See it in action on ideone.com