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.)

I am saving things like f(x) for definitions and shorthand for expressions. It means that I will never be able to have a function like x(x) but I am ok with that. — Ben Hernandez
– Ben Hernandez, Commented Jan 28, 2016 at 23:45
Another way: don't try to do it with a regex, and use a js parser to extract a graph of function calls... — anotherdev
– anotherdev, Commented Jan 28, 2016 at 23:46

Wiktor Stribiżew · Accepted Answer · 2016-01-28 23:49:39Z

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:

Negative lookahead is indispensable if you want to match something not followed by something else. When explaining [character classes][4], this tutorial explained why you cannot use a negated character class to match a q not followed by a u. Negative lookahead provides the solution: q(?!u). The negative lookahead construct is the pair of parentheses, with the opening parenthesis followed by a question mark and an exclamation point.

edited Jan 28, 2016 at 23:49

answered Jan 28, 2016 at 23:46

Wiktor Stribiżew

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2 Comments

Ben Hernandez Over a year ago

Ahhh, that makes sense, I need to read up on consumption, I am obviously not processing it correctly.

Ben Hernandez Over a year ago

Appreciate the link too!

Gavriel · Accepted Answer · 2016-01-28 23:44:43Z

0

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

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

answered Jan 28, 2016 at 23:44

Gavriel

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