Check whether a string starts with a digit

Use ^[0-9] to check for an ASCII digit at the start of a string. ^\d is convenient too, but the characters matched by \d depend on the language and regex options. Keep multiline mode off when ^ must refer only to the start of the whole input.

JavaScript

const value = '123abc';
console.log(/^[0-9]/.test(value)); // true

JavaScript's \d also means ASCII digits, including in Unicode mode. Do not add g to a reusable boolean-check regex unless you intend to manage its stateful lastIndex behavior.

Python

import re

value = '123abc'
if re.match(r'[0-9]', value):
    print('Starts with an ASCII digit')
else:
    print('Does not start with an ASCII digit')

re.match starts at the beginning, so the ^ is unnecessary here. For Python Unicode strings, \d also matches Unicode decimal digits; re.ASCII restricts that shorthand. re.search(r'\d', value) without an anchor can instead find a digit in the middle.

Java

import java.util.regex.Pattern;

String value = "123abc";
boolean startsWithDigit = Pattern.compile("[0-9]")
    .matcher(value)
    .lookingAt();

lookingAt() checks a prefix. String.matches() requires the whole string to match, so a pattern such as "^\\d.*" must consume the rest and can fail when it contains a line terminator unless the dot behavior is changed. Use a prefix matcher when that is the actual requirement.

Common variations

The nondigit pattern requires a character; an empty string does not match. Whitespace semantics also vary, and \s* may accept line breaks. Use [ \t]* if only spaces and tabs are allowed.

To require that the entire input consists of digits, prefer Python's re.fullmatch(r'[0-9]+', value) or Java's value.matches("[0-9]+"). In JavaScript, /^[0-9]+$/.test(value) checks the whole string when multiline mode is off. Do not assume this end-anchor behavior transfers unchanged to every engine: Python's $, for example, can also match before a final newline, which is why re.fullmatch is preferable there.

You can avoid regex for ASCII checks with value.length > 0 && value[0] >= '0' && value[0] <= '9' in JavaScript. Python's bool(value) and value[0].isdigit() has broader Unicode semantics, including some digits that \d does not match.

References: Python regular expressions, Java Pattern, and JavaScript character classes.