Regex to extract function notation with nested functions (javascript)

Question

I am trying to extract function notation from a string in javascript (so I can't use lookbehind), here are some examples:

• f(2) should match f(
• f(10)g(8) should match f( and g(
• f(2+g(3)) should match f( and g(
• f(2+g(sqrt(10))) should match f( and g(
• f(g(2)) should match f( and g(

Right now I am using

/\b[a-z]\([^x]/g

because I don't want to match when it is a string of letters (such as sqrt) only when there is a single letter then a parentheses. The problem I am having is with the last one in the list (nested functions). ( is not part of the \\b catches so it doesn't match.

My current plan is to add a space after every ( using something like

input = input.replace(/\([^\s]/g, '( ');

Which splits the nested function so that \\b comes into play [becomes f( g( 3))] but before I started messing with the input string, I thought I would ask here if there was a better way to do it. Obviously regex is not something I am super strong with but I am trying to learn so an explanation with the answer would be appreciated (though I will take any pointers that I can google myself too! I am not entirely sure of what to search for here.)

Answer 1

The point here is that [^x] is a negated character class that still matches , consumes the symbol after ( and it prevents overlapping matches. To make a check that the next character is not x , use a lookahead:

\b[a-z]\((?!x)
         ^^^^^

See regex demo

Perhaps, you want to fail a match only if a x is the only letter inside f() or g() :

\b[a-z]\((?!x\))

From Regular-expressions.info :

is indispensable if you want to match something not followed by something else. 是必不可少的。 When explaining [character classes][4], . 。 The negative lookahead construct is the pair of parentheses, with the opening parenthesis followed by a question mark and an exclamation point .

Answer 2

I think you just have to remove [^x]:

"f(g(2))".match(/\b[a-z]\(/g)
// ["f(", "g("]