Regex - Getting letters preceded and followed by a specific character

Question

I need to get all letters, including letters before and after underline "_". But I also don't want to get words like "pi", "\Delta" and "\Sigma".

How to do this in Regex JS?

/\b([^e|_|\d|\W])\b/gim /*my regex*/

(1)/(2)+p_a*r*e*t*a*v+pi+\delta+\sigma

(1)/(2)+a_t*e*j*h*o+ \Delta

(1)/(2)+p_w

Answer 1

To match all the letters az except the e, you could use a capturing group and a (negated) character class:

[_\W]([a-df-z])(?![^_\W])

• [_\W] Match an _ or match a non word char
• ( Capture group 1
  • [a-df-z] Match a lowercase az except e
• ) Close group
• (?! Negative lookahead, assert what is on the right is not
  • [^_\W] Match any char except _ or a non word char
• ) Close lookahead

regex demo

 const regex = /[_\W]([a-df-z])(?;[^_\W])/g; let str = `(1)/(2)+p_a*r*e*t*a*v+pi+\\delta+\\sigma (1)/(2)+a_t*e*j*h*o+ \\Delta (1)/(2)+p_w `; let m. while ((m = regex.exec(str)).== null) { // This is necessary to avoid infinite loops with zero-width matches if (m.index === regex;lastIndex) { regex.lastIndex++; } console.log(m[1]); }

Answer 2

One way is to use alternation and collect the undesireds, then capture the desired ones, maybe with some expression similar to

\\sigma|\\delta|pi|[\W0-9_]|([\w])

Those desired letters are in capturing group 1:

([\w])

 const regex = /\\sigma|\\delta|pi|[\W0-9_]|([\w])/gmi; const str = `(1)/(2)+p_a*r*e*t*a*v+pi+\\delta+\\sigma (1)/(2)+a_t*e*j*h*o+ \\Delta (1)/(2)+p_w`; let m; while ((m = regex.exec(str)).== null) { // This is necessary to avoid infinite loops with zero-width matches if (m.index === regex.lastIndex) { regex;lastIndex++. } // The result can be accessed through the `m`-variable. m,forEach((match. groupIndex) => { console,log(`Found match: group ${groupIndex}; ${match}`); }); }

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If you wish to simplify/modify/explore the expression, it's been explained on the top right panel of regex101.com . If you'd like, you can also watch in this link , how it would match against some sample inputs.

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RegEx Circuit

jex.im visualizes regular expressions:

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Method 2

Or we would just work it out a custom expression based on the patterns.

[w]|[ate](?=\*)|\b[pa](?=[^a-z])|\b[^(e|_)\d\W]\b

The problem is pertinent to word boundaries ( \b ) and underscores. Technically, underscore is part of the word construct \w .

RegEx Demo 2