Regex for a multiline string with PHP preg_match

Question

I'm trying to build a pattern for a multiline string, that must start with <?php or whitespace + <?php and NOT end with ?> or ?> + whitespace.

My attempt was /^\s?<\?php.*[^>]\s?$/s but it did not work. Also tried the negative lookahead - no use.

Any idea? Thanks in advance.

If you just trim() the string first, this gets a lot easier and could be done with simple str_pos calls — adeneo
– adeneo, Commented Jun 11, 2015 at 21:05
Not possible in my case. Here I just prepare patterns to pass them over to another script. — autarq
– autarq, Commented Jun 12, 2015 at 9:02

Wiktor Stribiżew · Accepted Answer · 2015-06-11 22:00:25Z

3

You can use

(?s)^\s*<\?php(?!.*\?>\s*$).*+$

See demo

Regex explanation:

(?s) - Enable singleline mode for the whole pattern for . to match newline
^ - Start of string
\s* - Optional whitespace, 0 or more repetitions
<\?php - Literal <?php
(?!.*\?>\s*$) - Look-ahead checking if the string does not end with ?>whitespace
.*+$ - Matches without backtracking any characters up to the string end.

The possessive quantifier (as in .*+) enables us to consume characters once, in 1 go, and never come back in search of possible permutations.

Possessive quantifiers are a way to prevent the regex engine from trying all permutations. This is primarily useful for performance reasons.

And we do not to use explicit SKIP-FAIL verbs then.

edited Jun 11, 2015 at 22:00

answered Jun 11, 2015 at 20:44

Wiktor Stribiżew

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4 Comments

anubhava Over a year ago

I think OP wants NOT end with ?> or ?>whitespace*

anubhava Over a year ago

Yes it works though I am not a fan of double negative lookbehind. However honestly I didn't downvote your answer someone else did.

Wiktor Stribiżew Over a year ago

@anubhava: Now it should

anubhava Over a year ago

Yes now it does and much better than 2 lookbehinds +1

anubhava · Accepted Answer · 2015-06-11 21:24:59Z

2

In PHP, you can use this regex:

'/^\s*<\?php(?:.(?!\?>\s*$))*$/s'

RegEx Demo

^\s*<\?php matches optional whitespaces and literal <?php at line start.
(?:.(?!\?>\s*$))* will match any 0 or more characters that don't end with ?>whitespace* using a negative lookahead.

Update: For efficiency this PCRE regex will perform faster than the previous one:

'/^\s*<\?php(?>.*\?>\s*$(*SKIP)(*F)|.*+$)/s'

RegEx Demo 2

(*FAIL) behaves like a failing negative assertion and is a synonym for (?!)
(*SKIP) defines a point beyond which the regex engine is not allowed to backtrack when the subpattern fails later
(*SKIP)(*FAIL) together provide a nice alternative of restriction that you cannot have a variable length lookbehind in above regex.

edited Jun 11, 2015 at 21:24

answered Jun 11, 2015 at 21:02

anubhava

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