Regex fails to recognize repeated instances of a specific pattern

Question

Regex fails to recognize repeated instances of a specific pattern

Currently, my goal is to create a JavaScript regex that can interpret instances of patterns like \123 and convert them into their corresponding ASCII values. For example, 65 should be replaced with A.

If the backslash \ itself needs to be included, it can be escaped as \\, so \\65 would appear as \65.

The challenge I'm encountering involves correctly parsing consecutive occurrences of the main pattern.
For instance, \65#\97 translates to A#a. However, \65\97 only changes to A\97.

Below is a key section of the code:

// Parse \65 but not \\65
input = input.replace(/(^|[^\\]{1})\\(\d{1,3})/g, function (m0, m1, m2) {
    var n = parseInt(m2);
    if (n < 256) {
        return m1 + String.fromCharCode(n);
    } else {
        return m0;
    }
});

You can explore an example demonstrating this behavior in a JSFiddle here.

I suspect the issue lies within the regex, although I haven't been able to pinpoint it yet.
I'm eagerly awaiting any insights or suggestions on resolving this matter :]

javascript regex

Answer 1

Answer №1

One way to solve this problem is by looking for a backslash followed by either another backslash or a sequence of digits. If it's a digit sequence, then you can check the value and make your substitution accordingly:

var input = "Do these: \\65\\97 \\66#\\97 But not: \\\\65\\\\97 Do this: \\\\\\65";
snippet.log("Before: " + input);
input = input.replace(/\\(\\|\d+)/g, function(m, c) {
  var val;
  if (c !== "\\") {
    val = +c;
    if (val >= 0 && val <= 255) {
      return String.fromCharCode(val);
    }
  }
  return m;
});
snippet.log("After: " + input);

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

Answer 2

One way to solve this problem is by looking for a backslash followed by either another backslash or a sequence of digits. If it's a digit sequence, then you can check the value and make your substitution accordingly:

var input = "Do these: \\65\\97 \\66#\\97 But not: \\\\65\\\\97 Do this: \\\\\\65";
snippet.log("Before: " + input);
input = input.replace(/\\(\\|\d+)/g, function(m, c) {
  var val;
  if (c !== "\\") {
    val = +c;
    if (val >= 0 && val <= 255) {
      return String.fromCharCode(val);
    }
  }
  return m;
});
snippet.log("After: " + input);

<!-- Script provides the `snippet` object, see http://meta.stackexchange.com/a/242144/134069 -->
<script src="http://tjcrowder.github.io/simple-snippets-console/snippet.js"></script>

Answer 3

Answer №2

You have the ability to capture both \\ and codes like \97 within a regular expression, while keeping \\ and codes above 255 unchanged:

let originalInput = "\\\\42This is \\97 \\116\\101st.\\\\256";

let modifiedInput = originalInput.replace(/(\\\\|\\\d{1,3})/g, function (match) {
  if (match != '\\\\') {  
    let number = parseInt(match.substr(1), 10);
    if (number < 256) {
       match = String.fromCharCode(number);
    }
  }
  return match;
});

document.write(modifiedInput);

Answer 4

You have the ability to capture both \\ and codes like \97 within a regular expression, while keeping \\ and codes above 255 unchanged:

let originalInput = "\\\\42This is \\97 \\116\\101st.\\\\256";

let modifiedInput = originalInput.replace(/(\\\\|\\\d{1,3})/g, function (match) {
  if (match != '\\\\') {  
    let number = parseInt(match.substr(1), 10);
    if (number < 256) {
       match = String.fromCharCode(number);
    }
  }
  return match;
});

document.write(modifiedInput);

Regex fails to recognize repeated instances of a specific pattern

Answer №1

Answer №2

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