JSFiddle - React, Tailwind, and code Playground

HTML

<div id='result'></div>

JavaScript

//All the length ending with 0* should get captured
var sampleStringWhichWorks = 'function checkLength(l, ls){\n' +
    'var line, lines, len, length01, linelength11;\n' +
    'line = l;\n' +
    'lines = ls;\n' +
    'len = line.length12;\n' +
    'length02 = lines.length13;\n' +
    'if(len==length03){\n' +
    'len--;\n' +
    'length04= length05 + 1;\n' +
    '}\n' +
    'linelength14 = len + length06;\n' +
    'return linelength15;\n' +
    '}\n';

var sampleStringWhichDoesNotWorks = 'function checkLength(l, ls){\n' +
    'var line, lines, len, length01, linelength11;\n' +
    'line = l;\n' +
    'lines = ls;\n' +
    'len = line.length12;\n' +
    'length02 = lines.length13;\n' +
    'if(len==length03){\n' +
    'len--;\n' +
//length05 does not get captured once removed the space
'length04=length05 + 1;\n' +
    '}\n' +
    'linelength14 = len + length06;\n' +
    'return linelength15;\n' +
    '}\n';

var result = $('#result');
var expression = /([^a-z.])length[\d]{2}([^a-z])/mig;

function parseCode(code) {
    var matches = match(expression, code);

    for (var counter = 0; counter < matches.length; counter++) {
        result.append((counter + 1) + '.' + ' ' + matches[counter]);
        result.append('<br/>');
    }
    result.append('----');
    result.append('<br/>');
}

function match(regex, string) {
    var matches = [],
        found;
    while (found = regex.exec(string)) {
        matches.push(found[0]);
        regex.lastIndex = found.index + 1;
    }
    return matches;
}


parseCode(sampleStringWhichWorks);
parseCode(sampleStringWhichDoesNotWorks);