replace_matches does not return {null}

<div class="WordSection1"><p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Times New
Roman","serif"">Hi,</span><pclass="MsoNormal"><span style="font-size:11.0pt;font-family:"Times New
Roman","serif""> </span><pclass="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Times New
Roman","serif"">Itried to order a text-column only by parts of the entries. Therefore I used regexp_matches(), but
unfortunatelyI am loosing rows.</span><p class="MsoNormal"><span lang="EN-US"
style="font-size:11.0pt;font-family:"TimesNew Roman","serif""> </span><p class="MsoNormal"><span lang="EN-US"
style="font-size:11.0pt;font-family:"TimesNew Roman","serif"">SELECT regexp_matches('abc','[0-9]+'), 
regexp_matches('123','[0-9]+');</span><pclass="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Times
NewRoman","serif""> </span><p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Times New
Roman","serif"">Doesnot return “{null}, {123}” but no result at all.</span><p class="MsoNormal"><span lang="EN-US"
style="font-size:11.0pt;font-family:"TimesNew Roman","serif""> </span><p class="MsoNormal"><span lang="EN-US"
style="font-size:11.0pt;font-family:"TimesNew Roman","serif"">Is this behavior expected? How can I work
around?</span><pclass="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Times New
Roman","serif""> </span><pclass="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Times New
Roman","serif"">Iam running postgres 8.4</span><p class="MsoNormal"><span lang="EN-US"
style="font-size:11.0pt;font-family:"TimesNew Roman","serif""> </span><p class="MsoNormal"><span lang="EN-US"
style="font-size:11.0pt;font-family:"TimesNew Roman","serif""> </span><p class="MsoNormal"><span lang="EN-US"
style="font-size:11.0pt;font-family:"TimesNew Roman","serif"">Andreas</span><p class="MsoNormal"><span lang="EN-US"
style="font-size:11.0pt;font-family:"TimesNew Roman","serif""> </span><p class="MsoNormal"><span lang="EN-US"
style="font-size:11.0pt;font-family:"TimesNew Roman","serif""> </span><p class="MsoNormal"><span
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