Replace only specific group in file form preg_replace

Question

I have txt file with content:

fggfhfghfghf

$config['website'] = 'Olpa';


asdasdasdasdasdas

And PHP script for replacing by preg_replace in file:

write_file('tekst.txt', preg_replace('/\$config\[\'website\'] = \'(.*)\';/', 'aaaaaa', file_get_contents('tekst.txt')));

But it doesn't work exactly what I want it to work.

Because this script replace whole match, and after change it looks like this:

fggfhfghfghf

aaaaaa


asdasdasdasdasdas

And that's bad.

All I want is to not change whole match $config['website'] = 'Olpa'; But to just change this Olpa

As you can see it belongs not to Group 2. of match information.

And all I want is to just change this Group 2. one specific thing.

to finally after script it will look like:

fggfhfghfghf

$config['website'] = 'aaaaaa';


asdasdasdasdasdas

Wiktor Stribiżew · Accepted Answer · 2017-07-05 21:06:52Z

11

You need to change your preg_replace to

preg_replace('/(\$config\[\'website\'] = \').*?(\';)/', '$1aaaaaa$2', file_get_contents('tekst.txt'))

It means, capture what you need to keep (and then use backreferences to restore the text) and just match what you need to replace.

See the regex demo.

Pattern details:

(\$config\[\'website\'] = \') - Group 1 capturing a literal $config['website'] = ' substring (later referenced to with $1)
.*? - any 0+ chars other than line break chars as few as possible
(\';) - Group 2: a ' followed with ; (later referenced to with $2)

In case your aaa actually starts with a digit, you would need a ${1} backreference.

answered Jul 5, 2017 at 21:06

Wiktor Stribiżew

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mickmackusa · Accepted Answer · 2017-07-11 10:24:49Z

I have a better, faster, leaner solution for you. No capture groups are required, it only requires careful attention to escaping the single quotes:

Pattern: \$config\['website'] = '\K[^']+

\K means "start the fullstring match here", this combined with the negated character class ([^']+) affords the omission of capture groups.

Pattern Demo (just 25 steps)

PHP Implementation:

$txt='fggfhfghfghf

$config[\'website\'] = \'Olpa\';


asdasdasdasdasdas';
print_r(preg_replace('/\$config\[\'website\'\] = \'\K[^\']+/','aaaaaa',$txt));

Using single quotes around the pattern is crucial so that $config isn't interpreted as a variable. As a result, all of the single quotes inside of the pattern must be escaped.

Output:

fggfhfghfghf

$config['website'] = 'aaaaaa';


asdasdasdasdasdas

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