Using JavaScript Regex to parse multi-line base64 encoded strings

Question

Using JavaScript Regex to parse multi-line base64 encoded strings

After receiving a MIME message, I am trying to extract the base64 encoded message from it, regardless of how many lines it spans.

The current regex pattern matches each individual line, but I want to group together multiple lines that contain matching base64 data. How can I achieve this?

var base64Regex = /^(?:[A-Za-z0-9+\/]{4})*(?:[A-Za-z0-9+\/]{4}|[A-Za-z0-9+\/]{3}=|[A-Za-z0-9+\/]{2}={2})$/gm

If there are other blocks of text within the MIME content (e.g., PGP signature), it will create separate matches for them as well, making it difficult to join all base64 matches together.

Ideally, I would like to modify the regex so that it captures everything from the first match to the next occurrence of `----------` and designates it as "match 1." Subsequent blocks of base64 data should be considered as separate matches ("match 2," etc).

To see an example of this behavior with 2 matches grouped into one, you can visit: https://regex101.com/r/32WjKa/1

javascript regex

Answer 1

Answer №1

Could this offer assistance?

var base64Pattern = /Content-Transfer-Encoding: base64([\s\S]*?)\s*?--/g;

Content-Transfer-Encoding: base64

- Indicating the beginning of the base64 encoded content.

[\s\S]*? - Represents the actual base64 encoded data, which can span multiple lines.

\s*? -- - Denoting the conclusion of the base64 encoded content.

g - Signifies the global flag to match all occurrences of the regular expression.

Answer 2

Could this offer assistance?

var base64Pattern = /Content-Transfer-Encoding: base64([\s\S]*?)\s*?--/g;

Content-Transfer-Encoding: base64

- Indicating the beginning of the base64 encoded content.

[\s\S]*? - Represents the actual base64 encoded data, which can span multiple lines.

\s*? -- - Denoting the conclusion of the base64 encoded content.

g - Signifies the global flag to match all occurrences of the regular expression.

Answer 3

Answer №2

Instead of searching for base64 characters specifically, it would be more effective to locate all characters (including newlines) within the HTTP payload's boundaries

By default, in Javascript regexes, the . does not match linebreaks even in multi-line mode. However, by using the /s flag, you can make . also match linebreaks.

Following this approach, you can easily eliminate line breaks after locating them with a simple replace()

const str = `--------------ra650umTsDNeI5lwXmFy5luF
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: base64

TG9yZW0gSXBzdW0NCg0KSGVyZSBpcyBzb21lIG1vcmUgdGV4dA0KDQpOb3cgb24gYSAzcmQg
bGluZQ0KDQoNClRoYW5rcw0KDQo=

--------------ra650umTsDNeI5lwXmFy5luF--`

const payload = str.match(/base64\n\n(.+)\n\n--------------.+/ms)[1].replace(/\n/g, '')

It might be advantageous to consider utilizing a module like body-parser as HTTP payloads similar to this are common practice.

Answer 4

Instead of searching for base64 characters specifically, it would be more effective to locate all characters (including newlines) within the HTTP payload's boundaries

By default, in Javascript regexes, the . does not match linebreaks even in multi-line mode. However, by using the /s flag, you can make . also match linebreaks.

Following this approach, you can easily eliminate line breaks after locating them with a simple replace()

const str = `--------------ra650umTsDNeI5lwXmFy5luF
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: base64

TG9yZW0gSXBzdW0NCg0KSGVyZSBpcyBzb21lIG1vcmUgdGV4dA0KDQpOb3cgb24gYSAzcmQg
bGluZQ0KDQoNClRoYW5rcw0KDQo=

--------------ra650umTsDNeI5lwXmFy5luF--`

const payload = str.match(/base64\n\n(.+)\n\n--------------.+/ms)[1].replace(/\n/g, '')

It might be advantageous to consider utilizing a module like body-parser as HTTP payloads similar to this are common practice.

Answer 5

Answer №3

Presented here are two approaches, one utilizing a regex .replace(), and the other using a .match() with positive lookbehind and positive lookahead:

const input = `--------------ra650umTsDNeI5lwXmFy5luF
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: base64

TG9yZW0gSXBzdW0NCg0KSGVyZSBpcyBzb21lIG1vcmUgdGV4dA0KDQpOb3cgb24gYSAzcmQg
bGluZQ0KDQoNClRoYW5rcw0KDQo=

--------------ra650umTsDNeI5lwXmFy5luF--`;

const regex1 = /^.*?Content-Transfer-Encoding: base64\s+(.*?)\s*---.*$/is;
let result1 = input.replace(regex1, '$1');
console.log(result1);

const regex2 = /(?<=Content-Transfer-Encoding: base64\s+).*?(?=\s*---)/is;
let result2 = input.match(regex2);
console.log(result2[0]);

Output:

TG9yZW0gSXBzdW0NCg0KSGVyZSBpcyBzb21lIG1vcmUgdGV4dA0KDQpOb3cgb24gYSAzcmQg
bGluZQ0KDQoNClRoYW5rcw0KDQo=

TG9yZW0gSXBzdW0NCg0KSGVyZSBpcyBzb21lIG1vcmUgdGV4dA0KDQpOb3cgb24gYSAzcmQg
bGluZQ0KDQoNClRoYW5rcw0KDQo=

Decoding the logic behind regex 1 for .replace():

^ -- start of line anchor
```
.*?Content-Transfer-Encoding: base64\s+
```
-- matches leading characters until base 64, including any whitespace
(.*?) -- capture group one: lazily captures all characters until:
\s*---.* -- whitespace, followed by ---, and everything afterward
$ -- end of line anchor
Utilizes flags is to enable case insensitivity and match newlines accordingly

Explanation of the second regex pattern used in .match():

```
(?<=Content-Transfer-Encoding: base64\s+)
```
-- positive lookbehind for literal text ...base64, along with associated whitespace
.*? -- non-greedy scan until:
(?=\s*---) -- positive lookahead for whitespace and ---
Flags utilized include is for case insensitivity and newline interpretation

Key Points:

Be aware that not all regex variations and browsing platforms support lookbehind functionalities, specifically Safari
It is safe to search for --- as an endpoint identifier since dashes are not included in base64 character sequences

Answer 6

Presented here are two approaches, one utilizing a regex .replace(), and the other using a .match() with positive lookbehind and positive lookahead:

const input = `--------------ra650umTsDNeI5lwXmFy5luF
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: base64

TG9yZW0gSXBzdW0NCg0KSGVyZSBpcyBzb21lIG1vcmUgdGV4dA0KDQpOb3cgb24gYSAzcmQg
bGluZQ0KDQoNClRoYW5rcw0KDQo=

--------------ra650umTsDNeI5lwXmFy5luF--`;

const regex1 = /^.*?Content-Transfer-Encoding: base64\s+(.*?)\s*---.*$/is;
let result1 = input.replace(regex1, '$1');
console.log(result1);

const regex2 = /(?<=Content-Transfer-Encoding: base64\s+).*?(?=\s*---)/is;
let result2 = input.match(regex2);
console.log(result2[0]);

Output:

TG9yZW0gSXBzdW0NCg0KSGVyZSBpcyBzb21lIG1vcmUgdGV4dA0KDQpOb3cgb24gYSAzcmQg
bGluZQ0KDQoNClRoYW5rcw0KDQo=

TG9yZW0gSXBzdW0NCg0KSGVyZSBpcyBzb21lIG1vcmUgdGV4dA0KDQpOb3cgb24gYSAzcmQg
bGluZQ0KDQoNClRoYW5rcw0KDQo=

Decoding the logic behind regex 1 for .replace():

^ -- start of line anchor
```
.*?Content-Transfer-Encoding: base64\s+
```
-- matches leading characters until base 64, including any whitespace
(.*?) -- capture group one: lazily captures all characters until:
\s*---.* -- whitespace, followed by ---, and everything afterward
$ -- end of line anchor
Utilizes flags is to enable case insensitivity and match newlines accordingly

Explanation of the second regex pattern used in .match():

```
(?<=Content-Transfer-Encoding: base64\s+)
```
-- positive lookbehind for literal text ...base64, along with associated whitespace
.*? -- non-greedy scan until:
(?=\s*---) -- positive lookahead for whitespace and ---
Flags utilized include is for case insensitivity and newline interpretation

Key Points:

Be aware that not all regex variations and browsing platforms support lookbehind functionalities, specifically Safari
It is safe to search for --- as an endpoint identifier since dashes are not included in base64 character sequences

Using JavaScript Regex to parse multi-line base64 encoded strings

Answer №1

Answer №2

Answer №3

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