Lexical Analysis

KS3 Computer Science

11-14 Years Old

48 modules covering EVERY Computer Science topic needed for KS3 level.

GCSE Computer Science

14-16 Years Old

45 modules covering EVERY Computer Science topic needed for GCSE level.

A-Level Computer Science

16-18 Years Old

66 modules covering EVERY Computer Science topic needed for A-Level.

GCSE Character Sets Resources (14-16 years)

  • An editable PowerPoint lesson presentation
  • Editable revision handouts
  • A glossary which covers the key terminologies of the module
  • Topic mindmaps for visualising the key concepts
  • Printable flashcards to help students engage active recall and confidence-based repetition
  • A quiz with accompanying answer key to test knowledge and understanding of the module

A-Level Character Sets Resources (16-18 years)

  • An editable PowerPoint lesson presentation
  • Editable revision handouts
  • A glossary which covers the key terminologies of the module
  • Topic mindmaps for visualising the key concepts
  • Printable flashcards to help students engage active recall and confidence-based repetition
  • A quiz with accompanying answer key to test knowledge and understanding of the module


In computer science, lexical investigation, lexing or tokenization is the way toward changing over a grouping of characters (as in a software engineer or a page) into an arrangement of tokens (strings with an appointed significance and thus distinguished). a developer that performs lexical investigation might be known as a lexer, tokenizer, or scanner, despite the fact that scanner is likewise a term for the initial step of a lexer.

Lexical analysis is a way to change  a grouping of characters into an arrangement of tokens

Lexical examination is the initial stage in planning the compiler. A lexeme is a grouping of characters remembered for the source software engineer as per the coordinating example of a symbol. The lexical analysis is executed to examine all the source code of the developer. The lexical analyzer is utilized to distinguish the token in the image table. a grouping of characters that can’t be digitized into a legitimate token is a lexical blunder. eliminating a character from the rest of the section is a helpful mistake recuperation strategy. lexical analyzer dissects the info software engineer while the analyzer performs parsing. encourages the cycle of lexical investigation and parsing by eliminating undesirable tokens. internet browsers utilize a lexical analyzer to organize and show a website page utilizing information parsed from JavaScript, HTML, CSS. The greatest burden of utilizing the lexical analyzer is that extra execution overhead is needed to create the lexer tables and build the symbols.      

Basic Terminologies for lexical analyzer


Lexeme is a succession of nature remembered for the source developer as indicated by the coordinating example of a token. it is simply a symbolic occurrence.


Symbol is a succession of nature that speaks to a part of data in the origin software engineering.


Format is a portrayal utilized by the token. On account of a catchphrase utilized as a symbol, the example is an arrangement of nature.

Lexical Errors

A succession of characters that can’t be digitized into a legitimate symbol is a lexical analyzer mistake. Significant realities about lexical analyzer mistake:

  •   lexical analyzer mistakes are not normal, yet they ought to be managed by a scanner.
  •  Spelling botches in identifiers, administrators and catchphrases are viewed as lexical blunders.
  • Usually, a lexical blunder is brought about by the presence of an illicit character, primarily toward the start of a symbol.


Here, are several different ordinary misstep recovery methodologies:

  •  Eliminates one nature from the remainder of the data
  •  In the craze mode, the dynamic characters are continually dismissed      until       
  • we show up at an overall surrounded symbol
  • By entering the missing nature into the remainder of the data
  •  Supplant a character with another character
  •  Render two consecutive characters

Now we discuss a difference between Lexical Analyzer & Parse

 Comparison of Lexical Analyzer & Parser

Lexical AnalyzerParser
Output Input projectCarry out linguistic structure examination
Recognize TokensMake a theoretical portrayal of the program
Addition symbols into Symbol TableBetter  image table sections
It creates lexical blundersIt create  parse tree of the program

 Lexical and Parser Separation

  • The straight forwardness of plan: It encourages the pattern of lexical assessment and the sentence structure examination by murdering bothersome symbols.
  •   For better compiler adequacy: Helps you to better compiler order.
  •  Explicit procedures can be applying to better the lexical assessment measure.
  • Transportability: simply the scanner needs to talk with the remainder of the world Higher mobility: input-contraption unequivocal attributes restricted to the lexer.

Role of Lexical Analyzer

Essential job: Scan a source program (a string) and split it up into little, significant units, called tokens.

Model: position: =initial rate*60;

Change into important units: identifiers, constants, administrators, and accentuation.

Different jobs:

  • Removal of comments
  • Case of conversion
  • Removal of white spaces

First in a compiler, the fundamental undertaking of the lexical analyzer is to peruse the info nature of the source code, bunch them into lexemes, and build an arrangement of tokens for every lexeme in the source program. the symbolic stream is sent to the parser for parsing. it is normal for the lexical analyzer to communicate with the image. at the point when the lexical analyzer finds a lexeme that comprises an identifier, it must embed this lexeme in the table of images.

Now and again, data about this cooperation is recommended in the figure. The connection is typically actualized by requesting that the analyzer call the lexical analyzer. The call, recommended by the get next Token order, makes the lexical analyzer read the characters from its contribution until it can recognize the following lexeme and produce the following token, which is gotten back to the analyzer. Since the lexical analyzer is the aspect of the compiler that peruses the source text, it can perform different undertakings other than distinguishing lexemes. One of these undertakings is to eliminate remarks and spaces (clear, newline, tab, and perhaps different characters used to isolate tokens in the section). Another errand is to relate the blunder messages created by the compiler with the source program. For instance, the lexical analyzer can monitor the quantity of newline characters that are shown, and afterwards it can relate a line number with every blunder message. In certain compilers, the lexical analyzer makes a duplicate of the source program with mistake messages set in the suitable spots. In the event that the source program utilizes a full-scale preprocessor, the lexical analyzer can likewise perform large scale development.

Lexical analyzer likewise performs beneath given assignments:

  • This distinguishes the token in the image table
  • Remove place and remarks from the source code.
  • Associate blunder messages with the source code.
  • Help to create macros in the event that they are in the source code
  • Study input nature from the source program

Output of Lexical analysis

To Understand the Output of Lexical Analysis, we discuss an example:

Here is a program

  Int maximum (Int a, Int b) {

             if (a > b)

             return a;

             else {

             return b;



The output generated by the lexical analyzer


To Understand the Output of Lexical Analysis, we discuss another example:

Int main () {

   Int a = 10;

    if (a < 2) {

      printf (“a is less than 2”);

   } else {

      printf (“a is not less than 2”);


   printf (“value of a is: %d\n”, a);

   return 0;


The output generated by the lexical analyzer


The lexical analyzer does not make the tokens of

  • Comment
  • Pre-processor directive
  • Pre-processor directive
  • Macro
  • Whitespace


There are some acceptable impacts of the lexical examination

  • The lexical analyzer technique is utilized by projects, for example, compilers that can utilize the information parsed by a software      engineer’s code to make ordered pairs executable code.
  • Used by internet browsers to arrange and show a site page utilizing parsed JavaScript, HTML and CSS information.
  • A separate lexical analyzer makes a specific and possibly more effective processor for the undertaking.

 There are some bad effects of the lexical analysis

  • You need to invest a ton of energy perusing the source program and parceling it into tokens
  • Some normal articulations are preferably hard to comprehend
  • More exertion is expected to create and focus the lexer and its symbolic depictions
  • Extra execution above is needed to create lexer tables and manufacture tokens

An examination of all coded scanners legitimately repudiates the disagreement that organized programming can be wasteful and the utilization of goto is important to accomplish quick code. Another fantasy about goto is that unmanaged code fundamentally meddles with program improvement. Outcomes are unique: initiating the primary degree of enhancement created a critical increasing speed for GNAT01 and GNAT03. To clarify this outcome, how about we investigate the created gathering program and tally the complete number of burden, supply and no guidelines.

Cannot read a characters twice

A key objective in the plan of scanners ought to be to “limit the occasions The character is influenced by the program.” Although it appears to be entirely self-evident, it is regularly hacked while utilizing a scanner generator. The issue happens when RE don’t depict a solitary lexical token however a class of tokens. Consider what the numbers resemble: first, the entered characters are contrasted with the relating characters RE and put away in a cradle generally supplied by the scanner generator. This stage it stops when it arrives at an end image. At that point the entire number is perused once more, this time from the support, to change over it to an interior portrayal. Nonetheless, straight-coded scanners can perform quick and relating transformations in a similar cycle. This eliminates the cradle and the second cycle e it likewise dodges re-investigating the specific succession of characters. Scanner Specifications dialects ought to permit semantic activities in RE as opposed to calling simply after an articulation completely coordinates, basically supporting an Attribution language structure previously utilized by numerous parser developers. To check the real impact, we adjust OPT1 by embedding the code in the principal store the digits in a cluster, at that point transform the number after the RE is completely perceived. This expanded the general uptime by around 1.4%. Considering the genuine and number literals just make up about 4% of all symbols in Ada (see Figure 6), this distinction would be huge for standard high recurrence dialects number literals.

Compiler design

A compiler deciphers code written in one language into another without altering the significance of the code. A compiler is additionally awaiting to make the objective code proficient and advanced as far as reality. The compiler plan standards give understanding into the interpretation and streamlining measure. The compiler venture covers the essential interpretation component and blunder location and comeback. It includes lexical, syntactic and semantic examination as a front-end and program age and enhancement as a back-end.

Why is it important to learn?

PCs are a reasonable blend of programming and apparatus. The apparatus is just a mechanical tool and its capacities are constrained by viable programming. We have discovered that each PC shell is composed of apparatus and programming. The material incorporates one language; that people can’t comprehend. That is the reason we compose programs in elevated level language that are more obvious and recall. These projects are then embedded into a progression of apparatuses and part of the working shell to get the ideal code that the machine can utilize. This is known as a language handling shell.

We have found that each PC shell is composed of apparatus and programming. The material incorporates language that people can’t comprehend. In this way, we compose programs in an elevated level language that are more obvious and recall. These projects are then embedded into a progression of instruments and part of the working shell to acquire the ideal code that the machine can utilize. This is called a language handling shell.

The raised language transforms into a twofold language in stages. A compiler is a program that changes over a noteworthy level language into a low-level processing development. Also, a building specialist is a program that changes over low level registering and develops into machine-level language.

We ought to at first see how a code capacity, using the c compiler, on a host tool.

  • The customer makes a program in C language (raised level language).
  • The C compiler assembles the program and causes an understanding of it into a get together program (low level language).
  • A developing specialist by then makes an understanding of the social affair program into machine (object) code.
  • An interface instrument is used to associate all bits of the program to be executed (executable machine code).

Before we plunge linear into compiler ideas, there are some different apparatuses that we have to comprehend that work intimately with compilers.

  •  Preprocessor
  • Interpreter
  • Assembler
  •   Linker
  •  Loader
  •  Cross-compiler
  • Source-to-source Compiler

Now we define these tools a little bit


A preprocessor, by and large viewed as a feature of the compiler, is an apparatus that creates contributions for compilers. It manages full scale preparation, increase, record incorporation, language expansion, and so forth.


A translator, similar to a compiler, deciphers an elevated level language into a low-level machine language. The thing that matters is standing out they read the source code or the section. A compiler peruses the entirety of the source code on the double, makes symbols, checks semantics, creates moderate code, runs the whole program, and can include numerous means. Rather, a mediator analyzes an assertion from the information, alters over it to transitional code, executes it, and afterward takes the following presentation all together


A building operator makes an understanding of low level processing develop programs into machine code. The yield of a building operator is known as an article record, which contains a blend of machine relation, similarly as the data expected to store those rules in memory.


Linker is a pc program that joins and unions various item records to make a workable document. These records may have been incorporated by independent constructing agents.


Loader is significant for the working shell and is managed for stacking executable archives into memory and executing them. Figure the size of a program (headings and data) and make memory space for it. Instate different registers to start execution.


A compiler that suddenly spikes in demand for the stage (An) and can create feasible code for the stage (B) is known as a cross compiler.

Source-to-source Compiler

A compiler that takes the source code of a single programming language and makes a grip of it into the source code of another programming language is known as a source-to-source compiler.

A compiler can exhaustively be divided into two phases subject to the way in which they request.

  1. Analysis Phase
  2. Synthesis Phase
Analysis PhaseSynthesis Phase
the compiler front-end, the compiler parsing stage peruses the source code, differentiates it into significant parts, and afterward checks for lexical, linguistic, and language structure mistakes. The examination stage creates a transitional portrayal of the source code and the table image, which must be passed to the combination stage as info.The back-finish of the compiler, the union stage creates the objective program using the middle portrayal of the source code and the image table.

A compiler can have numerous means and steps.

Pass: A pass alludes to the excursion of a compiler through the whole code.

Phase: An aggregation stage is a different period, which takes data from the past stage, measures it, and produces a yield that can be utilized as a contribution for the following stage. A stage can have a few periods.

The development cycle is a multiphase grouping. Each progression makes contribution from its past stride, has its own portrayal of the source code, and feeds its yield to the following compiler step. How about we comprehend the means of a compiler.

Which compiler is used for lexical analysis?

JavaCC is the standard Java compiler-compiler. Dissimilar to different devices introduced in this part, JavaCC is a parser and a scanner (lexer) generator in one. JavaCC takes only one information record (called the syntax document), which is then used to make the two classes for lexical examination, just as for the parser.

In JavaCC’s wording the scanner/lexical analyzer is known as the symbolic supervisor. What’s more, in actuality the created class that contains the symbolic administrator is called Parser Name Token Manager. Obviously, following the typical Java record name prerequisites, the class is put away in a document called ParserNameTokenManager.java. The Parser Name part is taken from the info record. What’s more, JavaCC makes a second class, called Parser Name Constants. That second class, as the name infers, contains meanings of constants, particularly token constants. JavaCC additionally creates a standard class called Token. That one is consistently the equivalent, and contains the class used to speak to tokens. One likewise gets a class called Parse Error. This is a special case which is tossed if something turned out badly.

It is conceivable to train JavaCC not to produce the Parser Name Token Manager, and rather give your own, manually written, token director. Typically – this holds for all the instruments introduced in this part – a transcribed scanner/lexical analyzer/token director is substantially more effective. Thus, on the off chance that you make sense of that your produced compiler gets excessively enormous, give the created scanner/lexical analyzer/token chief a decent look. Moving your own symbolic director is additionally convenient in the event that you have to parse double information and feed it to the parsing layer.

Since, notwithstanding, this part is tied in with utilizing JavaCC to produce a symbolic administrator, and not tied in with thinking of one by hand, this isn’t talked about any further here.

Characterizing Tokens in the JavaCC Grammar File

A JavaCC syntax document generally begins with code which is pertinent for the parser, and not the scanner. For basic punctuations records it seems to be like:

This is generally trailed by the definitions for tokens. These definitions are the data we are keen on in this section. Four various types, shown by four unique watchwords are perceived by JavaCC with regards to the meaning of tokens:


Customary articulations which indicate the tokens the symbolic chief ought to have the option to perceive.


SPECIAL_TOKENs are like TOKENS. Just, that the parser disregards them. This is valuable to for example determine remarks, which should be seen, however have no hugeness to the parser.  


Tokens (input information) which should be totally disregarded by the symbolic chief. This is usually used to overlook whitespace. A SKIP token despite everything separates different tokens. For example, on the off chance that one avoids a void area, has a token “else” characterized, and in the event that the information is “el se”, at that point the token isn’t coordinated.


This is utilized for a serious method, where a token is steadily manufactured. MORE tokens are placed in a cradle until the following TOKEN or SPECIAL_TOKEN matches. At that point all information, the aggregated token in the cradle, just as the last TOKEN or SPECIAL_TOKEN is returned. One model, where the use of MORE tokens is helpful, is developed where one might want to coordinate for some beginning string, discretionary information, and some end string. Remarks or string literals in many programming dialects coordinate this structure. For example, to coordinate string literals, delimited by “, one would not restore the main discovered ” as a token. Rather, one would aggregate more tokens, until the end ” of the string strict is found. At that point the total strict would be returned. See Comment Example for a model where this is utilized to examine remarks. Every one of the previously mentioned catchphrases can be utilized as frequently as wanted. This makes it conceivable to gather the tokens, for example in a rundown for administrators and another rundown for watchwords. All parts of a similar kind are converged as though only one area had been indicated.

Each particular of a symbol comprises the token’s emblematic name, and a customary articulation. On the off chance that the standard articulation coordinates, the image is returned by the symbolic administrator.                     


  1. Ada 95 Reference Manual. Intermetrics, Inc., 1995. ANSI/ISO/IEC-8652:1995.
  2. A.V. Aho and M.J. Corasick. Efficient String Matching: An Aid to Bibliographic Search. Communications of the ACM, 18(6):333-340, June 1975
  3. Alfred V. Aho, Monica S. Lam, Ravi Sethi, Jeffery D. Ullman Compilers : Principles, Technique and Tools, 2nd ed. PEARSON Education 2009.
  4. P. Bumbulis and D.D. Cowan. RE2C: A More Versatile Scanner Generator. ACM Letters on Programming Languages and Systems, 2(1-4):70–84, 1993.
  5. R.J. Cichelli. Minimal Perfect Hash Functions Made Simple. Communications of the ACM, 23:17–19, 1980.
  6. A.V. Aho, R. Sethi, and J.D. Ullman. Compilers. Addison-Wesley, 1986.
  7. J. Barnes. Programming in Ada 95. Addison Wesley, 1995.
  8. https://www.ques10.com/p/21837/explain-role-of-lexical-analyser/
  9. https://www.guru99.com/compiler-design-lexical-analysis.html
  10. https://www.javatpoint.com/compiler-tutorial
  11. https://www.tutorialspoint.com/compiler_design/compiler_design_types_of_parsing.htm
  12. https://en.wikibooks.org/wiki/Compiler_Construction/Lexical_analysis