preg_replace code evaluation with /e modifier

Question

I was looking for an alternative to eval() way to execute a code contained in a string variable, and noticed the preg_replace with the /e modifier. However, code is evaluated quite weirdly.

For instance, echo() leads to an error in php 5.3.6:

<?php
$code = 'echo( \'Hello, world!\'.PHP_EOL)';
preg_replace('/(.*)/e', $code, '' );
?>

Error message:

Parse error: syntax error, unexpected T_ECHO in /Users/.../test.php(4) : regexp code on line

Fatal error: preg_replace(): Failed evaluating code:
echo( 'Hello, world!'.PHP_EOL) in /Users/.../test.php on line 4

While print() works, and outputs a Hello, world!:

<?php
$code = 'print( \'Hello, world!\'.PHP_EOL)';
preg_replace('/(.*)/e', $code, '' );
?>

Also, it seems that out of several lines of code, only the first one is executed:

<?php
$t=1;
$code = '$t++;$t++;';
preg_replace('/(.*)/e', $code, '' );
echo $t;
?>

Outputs 2, not 3.

Question: is this code evaluation within PCRE in preg_replace() documented somewhere? Not found on php.net

Solution 1

The preg_replace /e eval does not evaluate statements, but just a single expression.

print()

Is an expression.

echo ...;

Is not.
And likewise anything that can be followed by a semicolon.

See http://www.php.net/manual/en/language.expressions.php for an rough overview.

Solution 2

Your better off creating an anonymous function and calling it using preg_replace_callback.