Regular Expressions 101

httpthe literal text http (case sensitive)

s?optionally matches the literal s (115₁₀ / 163₈ / 73₁₆) (case sensitive)

://stackoverflowthe literal text ://stackoverflow (case sensitive)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

com/users/the literal text com/users/ (case sensitive)

Group 1(\d{1,9})

\d{1,9}a digit, repeated between 1 and 9 times

/userthe literal text /user (case sensitive)

\1the text saved by group 1

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

Detailed match information will be displayed here automatically.

Search reference

Regular Expression

https?://stackoverflow\.com/users/(\d{1,9})/user\1

New

https://stackoverflow.com/users/12283851/user12283851 <== Valid
https://stackoverflow.com/users/11111111/user12283851 <== Invalid