regex - What is the best regular expression to check if a string is a valid URL?

Question

How can I check if a given string is a valid URL address?

My knowledge of regular expressions is basic and doesn't allow me to choose from the hundreds of regular expressions I've already seen on the web.

Answer 1

I wrote my URL (actually IRI, internationalized) pattern to comply with RFC 3987 (http://www.faqs.org/rfcs/rfc3987.html). These are in PCRE syntax.

For absolute IRIs (internationalized):

/^[a-z](?:[-a-z0-9+.])*:(?://(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:])*@)?(?:[(?:(?:(?:[0-9a-f]{1,4}:){6}(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|::(?:[0-9a-f]{1,4}:){5}(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){4}(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:(?:[0-9a-f]{1,4}:){0,1}[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){3}(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:(?:[0-9a-f]{1,4}:){0,2}[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){2}(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:(?:[0-9a-f]{1,4}:){0,3}[0-9a-f]{1,4})?::[0-9a-f]{1,4}:(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:(?:[0-9a-f]{1,4}:){0,4}[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:(?:[0-9a-f]{1,4}:){0,5}[0-9a-f]{1,4})?::[0-9a-f]{1,4}|(?:(?:[0-9a-f]{1,4}:){0,6}[0-9a-f]{1,4})?::)|v[0-9a-f]+.[-a-z0-9._~!$&'()*+,;=:]+)]|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3}|(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=])*)(?::[0-9]*)?(?:/(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@]))*)*|/(?:(?:(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@]))+)(?:/(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@]))*)*)?|(?:(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@]))+)(?:/(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@]))*)*|(?!(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@])))(?:?(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@])|[x{E000}-x{F8FF}x{F0000}-x{FFFFD}x{100000}-x{10FFFD}/?])*)?(?:#(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@])|[/?])*)?$/i

To also allow relative IRIs:

/^(?:[a-z](?:[-a-z0-9+.])*:(?://(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:])*@)?(?:[(?:(?:(?:[0-9a-f]{1,4}:){6}(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|::(?:[0-9a-f]{1,4}:){5}(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){4}(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:(?:[0-9a-f]{1,4}:){0,1}[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){3}(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:(?:[0-9a-f]{1,4}:){0,2}[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:){2}(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:(?:[0-9a-f]{1,4}:){0,3}[0-9a-f]{1,4})?::[0-9a-f]{1,4}:(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:(?:[0-9a-f]{1,4}:){0,4}[0-9a-f]{1,4})?::(?:[0-9a-f]{1,4}:[0-9a-f]{1,4}|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3})|(?:(?:[0-9a-f]{1,4}:){0,5}[0-9a-f]{1,4})?::[0-9a-f]{1,4}|(?:(?:[0-9a-f]{1,4}:){0,6}[0-9a-f]{1,4})?::)|v[0-9a-f]+.[-a-z0-9._~!$&'()*+,;=:]+)]|(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])(?:.(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])){3}|(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=])*)(?::[0-9]*)?(?:/(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@]))*)*|/(?:(?:(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@]))+)(?:/(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@]))*)*)?|(?:(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@]))+)(?:/(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@]))*)*|(?!(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@])))(?:?(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C0000}-x{CFFFD}x{D0000}-x{DFFFD}x{E1000}-x{EFFFD}!$&'()*+,;=:@])|[x{E000}-x{F8FF}x{F0000}-x{FFFFD}x{100000}-x{10FFFD}/?])*)?(?:#(?:(?:%[0-9a-f][0-9a-f]|[-a-z0-9._~x{A0}-x{D7FF}x{F900}-x{FDCF}x{FDF0}-x{FFEF}x{10000}-x{1FFFD}x{20000}-x{2FFFD}x{30000}-x{3FFFD}x{40000}-x{4FFFD}x{50000}-x{5FFFD}x{60000}-x{6FFFD}x{70000}-x{7FFFD}x{80000}-x{8FFFD}x{90000}-x{9FFFD}x{A0000}-x{AFFFD}x{B0000}-x{BFFFD}x{C00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