variable length masking with preg_replace

php, preg-replace, regex

Solution

Yes you can do it, (assuming that quotes are balanced) example:

$str = "TEST 'replace''me' ok 'me too'";
$pattern = "~[^'](?=[^']*(?:'[^']*'[^']*)*+'[^']*\z)|'~";    
$result = preg_replace($pattern, '-', $str);

The idea is: you can replace a character if it is a quote or if it is followed by an odd number of quotes.

Without quotes:

$pattern = "~(?:(?!\A)\G|(?:(?!\G)|\A)'\K)[^']~";
$result = preg_replace($pattern, '-', $str);

The pattern will match a character only when it is contiguous to a precedent match (In other words, when it is immediately after the last match) or when it is preceded by a quote that is not contiguous to the precedent match.

`\G` is the position after the last match (at the beginning it is the start of the string)

pattern details:

~             # pattern delimiter

(?: # non capturing group: describe the two possibilities
    # before the target character

    (?!\A)\G  # at the position in the string after the last match
              # the negative lookbehind ensure that this is not the start
              # of the string

  |           # OR

    (?:       # (to ensure that the quote is a not a closing quote)
        (?!\G)   # not contiguous to a precedent match
      |          # OR
        \A       # at the start of the string
    )
    '         # the opening quote

    \K        # remove all precedent characters from the match result
              # (only one quote here)
)

[^']          # a character that is not a quote

~

Note that since the closing quote is not matched by the pattern, the following characters that are not quotes can't be matched because there is no precedent match.

EDIT:

The `(*SKIP)(*FAIL)` way:

Instead of testing if a single quote is not a closing quote with `(?:(?!\G)|\A)'` like in the precedent pattern, you can break the match contiguity on closing quotes using the backtracking control verbs `(*SKIP)` and `(*FAIL)` (That can be shorten to `(*F)`).

$pattern = "~(?:(?!\A)\G|')(?:'(*SKIP)(*F)|\K[^'])~";
$result = preg_replace($pattern, '-', $str);

Since the pattern fails on each closing quotes, the following characters will not be matched until the next opening quote.

The pattern may be more efficient written like this:

$pattern = "~(?:\G(?!\A)(?:'(*SKIP)(*F))?|'\K)[^']~";

(You can also use `(*PRUNE)` in place of `(*SKIP)`.)

Problem

I am masking all characters between single quotes (inclusively) within a string using `preg_replace_callback()`. But I would like to only use `preg_replace()` if possible, but haven't been able to figure it out. Any help would be appreciated. This is what I have using `preg_replace_callback()` which produces the correct output: ``` function maskCallback( $matches ) { return str_repeat( '-', strlen( $matches[0] ) ); } function maskString( $str ) { return preg_replace_callback( "('.*?')", 'maskCallback', $str ); } $str = "TEST 'replace''me' ok 'me too'"; echo $str,"\n"; echo $maskString( $str ),"\n"; ``` Output is: ``` TEST 'replace''me' ok 'me too' TEST ------------- ok -------- ``` I have tried using: ``` preg_replace( "/('.*?')/", '-', $str ); ``` but the dashes get consumed, e.g.: ``` TEST -- ok - ``` Everything else I have tried doesn't work either. (I'm obviously not a regex expert.) Is this possible to do? If so, how?

Original source