Pattern matching with template literals in Typescript - Step-by-Step Execution

Concept Flow - Pattern matching with regex

Start with input string

↓

Define regex pattern

↓

Use match or regex to compare

↓

If matches

↓

Extract parts

↓

Use extracted values

This flow shows how a string is checked against a regex pattern, extracting parts if it matches.

Execution Sample

Typescript

const input = 'user:1234';
const pattern = /user:(\d+)/;
const match = input.match(pattern);
if (match) {
  console.log(`ID is ${match[1]}`);
}

This code matches a string against a pattern to extract a number after 'user:' and prints it.

Execution Table

Step	Action	Input	Pattern	Match Result	Extracted Value	Output
1	Define input string	user:1234
2	Define regex pattern		/user:(\d+)/
3	Match input with pattern	user:1234	/user:(\d+)/	Match found	1234
4	Check if match exists			Yes	1234
5	Print extracted ID				1234	ID is 1234

💡 Matching completes after extracting number and printing output.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
input	undefined	user:1234	user:1234	user:1234
pattern	undefined	undefined	/user:(\d+)/	/user:(\d+)/
match	undefined	undefined	["user:1234", "1234"]	["user:1234", "1234"]

Key Moments - 2 Insights

Why does match[1] contain '1234' instead of the whole string?

match[0] is the full matched string 'user:1234', while match[1] is the first captured group (\d+), which is '1234' as shown in execution_table step 3.

What happens if the input does not match the pattern?

The match variable becomes null, so the if condition fails and no extraction or output happens, as implied by the 'No' branch in concept_flow.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'match' after step 3?

A ["user:1234", "1234"]

B null

C "user:1234"

D "1234"

💡 Hint

Check the 'Match Result' and 'Extracted Value' columns in step 3 of execution_table.

At which step does the program print the output?

A Step 3

B Step 4

C Step 5

D Step 2

💡 Hint

Look at the 'Output' column in execution_table to find when output occurs.

If the input was 'admin:5678', what would happen to the 'match' variable?

A It would match and extract '5678'

B It would be null because pattern expects 'user:' prefix

C It would throw an error

D It would match the whole string

💡 Hint

Refer to the pattern in execution_sample and how match works in execution_table step 3.

Concept Snapshot

Pattern matching with regex uses regex to check if a string fits a pattern.
Use capturing groups (parentheses) to extract parts.
match() returns an array if matched, null if not.
match[0] is full match, match[1..] are captured groups.
Use if-check to safely extract and use values.

Full Transcript

This visual trace shows how regex pattern matching works in TypeScript. We start with an input string 'user:1234' and a regex pattern /user:(\d+)/. The program uses input.match(pattern) to check if the input fits the pattern. If it matches, match is an array where match[0] is the full string and match[1] is the captured number '1234'. The program then prints 'ID is 1234'. If the input does not match, match is null and no output occurs. Variables like input, pattern, and match change as the program runs, shown step-by-step. Common confusions include why match[1] holds the number and what happens if no match is found. The quiz tests understanding of match values and program flow. This helps beginners see exactly how pattern matching extracts data from strings.