0708fee0.ff9fc39e.js

(window.webpackJsonp=window.webpackJsonp||[]).push([[19],{117:function(e,t,r){"use strict";r.r(t),r.d(t,"frontMatter",(function(){return i})),r.d(t,"metadata",(function(){return l})),r.d(t,"rightToc",(function(){return p})),r.d(t,"default",(function(){return s}));var n=r(2),o=r(6),a=(r(0),r(469)),i={title:"Looking ahead"},l={id:"tutorial/08_look_ahead",isDocsHomePage:!1,title:"Looking ahead",description:"notFollowedBy",source:"@site/docs/tutorial/08_look_ahead.md",permalink:"/docs/next/tutorial/08_look_ahead",editUrl:"https://github.com/parsica-php/parsica/tree/main/docs/tutorial/08_look_ahead.md",version:"next",sidebar:"docs",previous:{title:"Recursion",permalink:"/docs/next/tutorial/07_recursion"},next:{title:"Errors and labels",permalink:"/docs/next/tutorial/09_errors_and_labels"},latestVersionMainDocPermalink:"/docs"},p=[{value:"notFollowedBy",id:"notfollowedby",children:[]}],c={rightToc:p};function s(e){var t=e.components,r=Object(o.a)(e,["components"]);return Object(a.b)("wrapper",Object(n.a)({},c,r,{components:t,mdxType:"MDXLayout"}),Object(a.b)("h2",{id:"notfollowedby"},"notFollowedBy"),Object(a.b)("p",null,"Say you want to match the ",Object(a.b)("inlineCode",{parentName:"p"},"print")," keyword in a programming language. You can express that with the ",Object(a.b)("inlineCode",{parentName:"p"},'string("print")')," parser, but it will match more than you'd like:"),Object(a.b)("pre",null,Object(a.b)("code",Object(n.a)({parentName:"pre"},{className:"language-php"}),'<?php\n$print = string("print");\n\n$result = $print->tryString("print(\'Hello World\');");\nassertEquals("print", $result->output());\n\n$result = $print->tryString("printXYZ(\'Hello World\');");\nassertEquals("print", $result->output()); // oops!\n')),Object(a.b)("p",null,'As you can see, "printXYZ" also results in "print", but it wasn\'t our intention, because "printXYZ" is not a valid keyword.'),Object(a.b)("p",null,"We can solve it by using the ",Object(a.b)("inlineCode",{parentName:"p"},"notFollowedBy")," combinator."),Object(a.b)("pre",null,Object(a.b)("code",Object(n.a)({parentName:"pre"},{className:"language-php"}),'<?php\n$print = keepFirst(string("print"), notFollowedBy(alphaNumChar()));\n$result = $print->run(new StringStream("printXYZ(\'Hello World\');"));\nassertTrue($result->isFail());\n')),Object(a.b)("p",null,"There's a fluent interface as well:"),Object(a.b)("pre",null,Object(a.b)("code",Object(n.a)({parentName:"pre"},{className:"language-php"}),'<?php\n$print = string("print")->notFollowedBy(alphaNumChar());\n$result = $print->run(new StringStream("printXYZ(\'Hello World\');"));\nassertTrue($result->isFail());\n')),Object(a.b)("p",null,"In practice, we'll have a lot more keywords than just the one. 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