Regex

You might say

AI gave me a regex I cannot read. Should I use it?

A notation for describing text to match, search, or constrainA regular expression describes text patterns for matching, searching, or constraining an input, such as \d for digits and + for one or more. This page uses JavaScript regex syntax to constrain a numeric-text field; a match only means the text fits this rule, not that a phone number or email is valid.

Know first

Validation

Regex

One regex: click inputs and see pass or reject

Validation rule/^\d+$/

Selected: 48120 → passes becausethe whole string is digits

This checks numeric text, not phone validity; acceptance should cover rejects such as spaces, letters, and empty input.

Three things to read in a regex

Common symbol meanings: In this example, \d is one digit and + means one or more; ^ and $ mark the pattern's boundaries; brackets can list allowed characters.

Test it with real inputs: To judge whether a regex fits, test both matching and non-matching boundary inputs, such as digits, symbols, and empty values.

Acceptance must cover rejects: Testing only passing cases is not acceptance; confirm results for inputs that should be rejected, including spaces, letters, and empty values.

Quick checkChoose the best answer

AI uses /^\d+$/ to constrain a text field. What does this regex accept?

A match means this is a valid phone numberAny text at allOne or more digits only; letters and symbols do not match

You can say this to an AI Agent

Use /^\d+$/ to constrain a numeric-text field. Test digits, letters, spaces, and empty input; confirm the pass and reject results match the rule. Do not present it as phone-number validation.

Learn next

1Validation→2Form→3Field→

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