What is the method for creating a regex pattern that captures a single symbol under specific conditions?

Question

What is the method for creating a regex pattern that captures a single symbol under specific conditions?

I am in need of a JavaScript regex that can specifically select a single character with one condition: it must not be followed or preceded by a particular specified character.

The target character is /, but I want to select it only if it is not immediately adjacent to the character a.

For example:

str = "I Like this/ and a/ basketball is round a/a ups.Papa/ tol/d /me tha/t";
    
myregex = ????
var patt = new RegExp(myregex);
var res = patt.split(str);

The desired result should look like this:

res[0] = "I Like this"
res[1] = " and a/ basketball is round a/a ups.Papa/ tol"
res[2] = "d "
res[3] = "me tha/t"

The regex pattern needed is something along the lines of:

(if not a then)(\/)(if not a then)

I have attempted to create the regex using: [^a](\/)[^a], however, this also selects characters directly next to the / such as s/ and l/d, rather than just isolating the instances where the a character is present. My goal is to exclude selections of characters beside the /.

javascript regex

Answer 1

Answer №1

Separate using /(?<!a)\//g

var output = "I Like this/ and a/ basketball is round a/a ups.Papa/ tol/d /me tha/t".split(/(?<!a)\//g);
console.log(output);

Explanation

/(?<!a)\/ matches / that doesn't come after a

Note

Works on chrome, does not work in FF according to @SoulReaver's comments.

Edit

Alternatively, you can also split by /

var output = "I Like this/ and a/ basketball is round a/a ups.Papa/ tol/d /me tha/t".split(/\//g);
output = output.reduce( function(a,c){
  var lastItem = a.slice(-1)[0];
  //console.log(lastItem)
  if ( lastItem && lastItem.slice(-1) == "a" )
  {
     a.pop();
     a.push(lastItem.concat("/").concat(c));
  }
  else
  {
     a.push(c);
  }
  return a;
}, [])
console.log(output);

Edit 2

If you want to ignore a on both sides of /, utilize the previous answer (using reduce), simply include the look-ahead in split

var output = "I Like this/ and a/ basketball is round a/a ups.Papa/ tol/d /me tha/t".split(/\/(?!=a)/g);

Answer 2

Separate using /(?<!a)\//g

var output = "I Like this/ and a/ basketball is round a/a ups.Papa/ tol/d /me tha/t".split(/(?<!a)\//g);
console.log(output);

Explanation

/(?<!a)\/ matches / that doesn't come after a

Note

Works on chrome, does not work in FF according to @SoulReaver's comments.

Edit

Alternatively, you can also split by /

var output = "I Like this/ and a/ basketball is round a/a ups.Papa/ tol/d /me tha/t".split(/\//g);
output = output.reduce( function(a,c){
  var lastItem = a.slice(-1)[0];
  //console.log(lastItem)
  if ( lastItem && lastItem.slice(-1) == "a" )
  {
     a.pop();
     a.push(lastItem.concat("/").concat(c));
  }
  else
  {
     a.push(c);
  }
  return a;
}, [])
console.log(output);

Edit 2

If you want to ignore a on both sides of /, utilize the previous answer (using reduce), simply include the look-ahead in split

var output = "I Like this/ and a/ basketball is round a/a ups.Papa/ tol/d /me tha/t".split(/\/(?!=a)/g);

Answer 3

Answer №2

Currently, using regex in JavaScript to implement lookbehind is not feasible due to lack of support in all browsers. A workaround could be using /\/(?!a)/g, which matches any forward slash (/) not followed by the letter 'a'. The complete solution, with lookbehind functionality, would require incorporating gurvinder372's suggestion: /(?<!a)\/(?!a)/g

UPDATE: It appears that this feature may soon become available in other browsers as well. TC39, the entity responsible for setting ECMAScript standards, has moved the proposal to include RegEx lookbehind to stage 4 - finalized on January 24th (with revisions made on January 26th). More details about this can be found here, along with current browser compatibility on this page. Presently, only Chrome and Opera (which share the same engine) support lookbehind.

Regarding the timeline for when this feature will be universally available, information on Safari and iOS browser release schedules is limited. However, in the case of Firefox, the latest version - Firefox 60 (Nightly) - does not support lookbehind. This version is set to become the next ESR release in May. If you prioritize ESR versions over constantly updating to newest releases, you may have to wait until the following ESR release, which should likely include this feature.

UPDATE 2:

Fast forward 2 years, it's time for an update. In the release notes for Firefox 78 on June 30, 2020, lookbehind regex support has finally been added. Additionally, this release is part of the ESR lineup. According to data from Can I Use, Safari remains the only major browser yet to adopt this feature.

Answer 4

Currently, using regex in JavaScript to implement lookbehind is not feasible due to lack of support in all browsers. A workaround could be using /\/(?!a)/g, which matches any forward slash (/) not followed by the letter 'a'. The complete solution, with lookbehind functionality, would require incorporating gurvinder372's suggestion: /(?<!a)\/(?!a)/g

UPDATE: It appears that this feature may soon become available in other browsers as well. TC39, the entity responsible for setting ECMAScript standards, has moved the proposal to include RegEx lookbehind to stage 4 - finalized on January 24th (with revisions made on January 26th). More details about this can be found here, along with current browser compatibility on this page. Presently, only Chrome and Opera (which share the same engine) support lookbehind.

Regarding the timeline for when this feature will be universally available, information on Safari and iOS browser release schedules is limited. However, in the case of Firefox, the latest version - Firefox 60 (Nightly) - does not support lookbehind. This version is set to become the next ESR release in May. If you prioritize ESR versions over constantly updating to newest releases, you may have to wait until the following ESR release, which should likely include this feature.

UPDATE 2:

Fast forward 2 years, it's time for an update. In the release notes for Firefox 78 on June 30, 2020, lookbehind regex support has finally been added. Additionally, this release is part of the ESR lineup. According to data from Can I Use, Safari remains the only major browser yet to adopt this feature.

What is the method for creating a regex pattern that captures a single symbol under specific conditions?

Answer №1

Answer №2

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